Install XCode. A bit of overkill for that one button, yes.
Open Safari's package contents (control-click and select Show Package Contents)
Navigate into Contents/Resources/English.lproj
Open ToolbarItems.nib (double clicking should open with Interface Builder)
In the 'ToolbarItems.nib (English)' window, double click on 'First Responder.'
Press Command-Shift-I to bring up the Inspector window, if it's not already open
In the Inspector window, press Command-1 to make sure you're looking at 'Attributes.' Click on Add, and create a new item called newWindow.
In the 'Toolbar Items' window, click on one of the 'torn pictures' and press Command-Shift-I to bring up the Inspector window if it's not already open.
Select the one nearest the Address bar (this should correspond to the placement in the Customise Toolbar menu); this is the 'Add Bookmark' button.
Press Command-2 or choose Connections. Select 'newWindow' and click 'Change Action' or simply double-click on it; there should be a round pushbutton icon next to it now.
Save, quit and test.
This is a set of directions modified from here
This is a lot to go through instead of just pressing [Command+N] on the keyboard while Safari is open.