If you are referring to active cards that actually appear as a serial port, sure, it is easy (much easier than passive cards).  Before I had DSL, I did exactly that using a USR Courier I-modem Internal card.  As the card looked just like a vanilla modem, it installed and ran with no special mucking around at all.  The only thing I did was configure it in a separate box (SPIDs, call type, etc.) before doing the e-smith install.  Worked like a charm.