This is the curse of FCP right now. Only one frame rate at a time. Mixing material from different rates is enough work to make you consider yak herding as a possible career change.
The only way I personally know to add CORRECT pulldown to bring it up to 29.97 is to run the files through After Effects.
You bring the files in, put it in a 23.98 comp, and then tell the export module to use field rendering, and chose a pattern. AE then is smart enough to figure out what you're after, and changes the output frame rate to 29.97. Time intensive, to say the least.
People keep telling me you can have Cinema Tools add pulldown as well, but I've never tried. Conforming, btw, would be the act of making the 23.98 simply playback at 29.97. Way different than adding pulldown.