You need the white attacks (autoattacks) to get rage. Slam (actually no specials) give you any rage. So the best way is to do slam just after a white attack has landed, as you say you then prolong your swing time by 1.5+ sec (without imp slam).

Slam guide by Caesar: http://www.honorbound.se/Slamdps2.pdf

To Funch, this thread (and spreadsheet) is old, you should post your comment in the new 2.3 dps spreadsheet thread. (edit: nvm you already did)

e: Nothing to see here you crazy thread necro guys.