Not a problem, and to answer your question, most of the third party upgrades, such as those from Sonnet, work fine in the Kansas machines. The upgrades that came out before the Kansas was released probably won't work. And I think there were a few on which you actually had to change a DIP switch setting if it was going in a Kansas.
Ultimately, you need to find a compatibility/system-requirements list for your upgrade to know whether it will work in a Kansas machine. For that matter, some of the upgrades don't work in the Catalyst-based Power Computing machines. Those require some trickiness with a couple of CPU signals.