Oh I forgot you can do Avid DNx in MPEG Streamclip.
Import your clip to Mpeg streamclip
Export to quicktime
select Avid DNxHD codec – select the correct resolution in the options e.g. 1080p/29.97
If you’re working in a 24p project you’re going to have to import any footage of a different frame rate into it’s own separate project, so 29.97 footage into a 29.97 project. Then open the bin with that footage from within your 24p project. When you drop the 29.97 footage on your timeline it will have a dot to show you that it’s got a motion effect on it. If you click on the clip and open the motion effect editor you can promote the effect in order to change the type to fluid motion (which will give the nicest playback)
That’s the method in media composer, I don’t know what features are available in xpress pro.
Best
Paddy