There are several replacement launchers in the market that will allow this...Go Launcher EX, ADW.EX, Launcher Pro, etc...
However, with these launchers you will loose some of the MotoBlur widgets like "favorite contacts"...if you install multiple launchers, also check out Moto Home Fix (app in market) and HomeSmack (app in market)...the first one will allow you to set the stock launcher as home even with additional launcers installed and the second app is an easy way to switch between multiple launchers.
So with these launcher apps do you lose the look and functionality of the original apps or can I utilize adding extra home screens and leaving everything else alone?