One way is to first complete all of the hierarchy levels you intend to illustrate in the Warnier/Orr Diagram even though the elements to the right of the left/open curly braces may look oddly out of place due to being centered. When finished, save an extra copy of the Word document for experimentation.
Download and install a copy of the excellent open source code text editor, Notepad++. Select the entire Warnier/Orr Diagram "equation" from one of your Word document copies, then copy and paste the Warnier/Orr diagram into a running instance of Notepad++. The Warnier/Orr Diagram "equation" will appear in linear text format in Notepad++.
Use the Notepad++ Search and Replace functionality to find every instance of the left parentheses, ( , and replace them with a left parentheses immediately followed by an ampersand without any space between them: (&.
Afterwards, use the Notepad++ Search and Replace functionality to the find every instance of the "At" symbol, @ , and replace them with an "At" symbol immediately followed by an ampersand without any space between them: @&
Select the entire updated linear format of the Warnier/Orr Diagram "equation", then copy and paste it over the original Warnier/Orr Diagram "equation" in one of the copies of the Word document. The overwritten Warnier/Orr "equation" will display in linear text format in the Word document rather than in the former diagram format.
Right click on the linear format text you just pasted and select, "Professional", from the pop-up menu. The Warnier/Orr Diagram "equation" should now appear in graphical form again, but all of the elements to the right of every left/open curly brace should be left aligned.
If you add more hierarchy levels to the Warnier/Orr Diagram "equation" later, and if you want the elements to the right of the newly added left/open curly braces to be left-aligned following the instructions above, you'll first need to find every instance of existing ampersands and replace them with nulls/nothing/blank (NOT any white space or tab) using the Notepad++ Search and Replace functionality. This is in order to avoid putting two ampersands, && , next to each other in the linear format of the Warnier/Orr Diagram if you follow the earlier steps above.
Explaining/reading the steps takes much longer than the actual effort. Doing it once will show it isn't that complex and can be done quickly. Using Notepad++ and other similar text editors allow one to create a macro to remove/add the ampersands with one click or key-combination.
You can also try to manually enter an ampersand, & , in front of each of the centered elements to the right of the left/open curly brace within the Word document. However, it may not always work as expected and can be less reliable than the suggested select/copy/paste/search/replace steps.
An alternative way to do the left alignment is to insert a vertical matrix into one of the empty placeholder/case/elements to the right of a left/open curly brace. The matrix can then be left-aligned by right clicking on the centered matrix item and selecting "Left" from the "Column Alignment" option from the pop-up menu. However, that will need to be done for each left/open curly brace. Inserting new matrix elements (rows) to the right of the left/open curly brace requires right clicking and selecting from the "Insert" options from the pop-up menu.
If you plan on creating/maintaining large Warnier/Orr Diagrams, I would suggest using the open source Freeplane mind-mapping software instead of the Equation functionality in the MS Office products. Don't use Freemind, it doesn't work as well for this purpose. There's a way to configure Freeplane so that it can support the appearance of Warnier/Orr Diagrams very well. I'd say they can look as good as they appear in the Ken Orr, David Higgins, Kirk Hansen, or David Brackett books. However, the Warnier/Orr Diagram will use left/open square brackets rather than left/open curly braces.
One great advantage in using Freemind is that you can close/hide/collapse any of the brackets in very large Warnier/Orr Diagrams that aren't of interest at the moment, making the design easier to understand at a higher level when presenting them, until you need to dig in deeper into the details of the closed/collapsed brackets.
Another powerful advantage in using Freeplane is that the Warnier/Orr Diagram can be edited/modified/expanded/navigated very easily by keyboard alone, very much like a text editor. That can help maintain flow/concentration as you use the Warnier/Orr Diagram to analyze processes/systems.
Once you have a Warnier/Orr Diagram configured the way you like it in Freeplane (such as using conditional styles [horizontal edge, hide edge,...etc], and a simple script to add an empty bracket mapped to a function key, AND/OR symbols mapped to function keys...etc), you can make a template of the Warnier/Orr Diagram mind-map file and use it for future Warnier/Orr Diagrams.