Final Cut Pro lets you drop anything into it’s timeline.  Just because you can doesn’t mean you should though.  The function is good for offline editing but when you are ready to finish your film all the media should match in frame-rate, resolution and codec.  Editing 23.98 material in a 29.97 interlaced timeline will get failed by a network because Final Cut Pro does not add proper pull-down in the timeline.  It merely duplicates every fourth frame.  The photo here shows you what settings to use in After Effects to add a 2:3 pull-down to your shots, converting a 23.98 progressive clip to an interlaced 29.97 shot.  Just remember to place your media into a 23.976 composition first.  When you choose “Lower Field First” the frame-rate automatically changes to 29.97.  We use “Lower Field First” when making NTSC SD and “Upper Field First” for PAL SD and HD interlaced material.  Now cut the rendered clip back into your timeline and finish your film.