I have a .rar file, (already compressed) and I need to split it. Can this be done through the WinRAR UI?
Yes, you can do it using WinRAR.
Open WinRAR window and navigate to the folder which your file is located in that WinRAR window.
Right click on your RAR file and select "Add files to archive" (alternatively you can press Alt+A).
In the opened window, give a new name to your to-be-split file (e.g. Archive_2.rar). Then, under "Compression Method", select "Store". Input your desired value inside "Split to volumes, bytes" box, click OK and you're done.