I'm no fan of Origin myself, to be honest. But your choices other than switching to an Origin install would be acquiring a new IP disc, or one of a later released EP/SP (the only two would be Movie Stuff and ITF) and install that so it becomes the required disc; or repurchase the game and all of your packs on Steam, then replace your game install with an all-Steam one (they can't be mixed and matched).
This is assuming you are on Windows, the Mac version of the game is handled a bit differently.
I guess another choice would be to uninstall IP and use the disc of what then becomes your most recently released pack if you still have it. But that may not be a very attractive option, especially if you have ongoing games that have seen the IP expansion that you want to keep playing forward.
