actually you can! I just tested it using a liverpool reserves.... put them in
prem instead of man u

lol.
See this picture for proof:-
http://i20.photobucket.com/albums/b2.../liv-resrv.jpg
How to do it... very simple
Goto the club in the official editor that you want to make a B team. Then edit in the menu, & add team. Then choose what the extention should be eg. B or reserves etc.
I chose reserves for liverpool as that was what it was called before. I dont know what would happen if I called it B team. So there you go, it is possible.
This gave me an idea, turn conference south and north into B team leagues....