Since FCE doesn't deal with 24fps I'm assuming you mean 24p as it comes fro mthe camera. (will pulldown added). In that case you shoudl be able edit 29.97 footage with your 24p footage. 29.97 will have a video look and 24p will have a film look.
If you want to truly convert 29.97 to 24p (and then add pulldown) you have to purchase special filters (Natress?) or other program.