I was given a DNxHD file in an MXF wrapper and need to work with it in FCP X. As far as I know, FCP X does support DNxHD files if it is rewrapped as QT/MOV and if the Avid QuickTime codec LT plug-in is installed. Whereas, DNxHD MXF files are not supported by Final Cut Pro. Converting DNxHD to ProRes is the best option I’ve ever worked out. Now I will share a full guide on how to transcode DNxHD to ProRes 422, ProRes 4444 for FCP native editing on Mac OS X.

How can we have a fast DNxHD to ProRes conversion? Any software can process the DNxHD(mxf) to Apple ProRes conversion? Based on my multiple testing, I found Brorsoft Video Converter for Mac would be the best tool to transcode DNxHD to ProRes 422, ProRes 4444.
With this professional DNxHD to ProRes conveter app, you can easily convert all DNxHD files to ProRes 422, ProRes 422 HQ or ProRes 4444 for using in Final Cut Pro X/7/6. Besides, the Mac video converter can convert MKV, AVI, WMV, MPG, VOB for editing in Final Cut Pro, Final Cut Express, iMovie, Avid MC, Adobe Premiere Pro etc. Some simple video editing functions like trim, crop, add watermark are also provided for you. Just download it and follow the tutorial below to convert DNxHD to Apple ProRes for FCP.
Tutorial: Transcode DNxHD to Apple ProRes on Mac
Step 1. Install the DNxHD to ProRes Converter program on Mac and run it. Click "Add" button on the top of the main interface to import video to the program. It supports batch conversion and joining multiple video clips into one file.

Step 2. Click the "Format" bar and select Final Cut Pro> Apple PreRes 422(*.mov) as output format. This format is specially designed for FCP, so that you can edit your original DNxHD video in Final Cut Pro perfectly.

Tip: You also can click “Settings” to set video frame rate, video size and Video Bitrate as you want.
Step 3. Click the “Convert” icon; it will start converting DNxHD to ProRes .mov for FCP 7/X on Mac immediately.
Once the conversion process shows in 100%, launch Final Cut Pro, choose File > Import > Files…, and browse to where you save the converted DNxHD clips to. Now you can start fun away with editing DNxHD videos in Final Cut Pro X or FCP 7/6 without any problem. Enjoy.
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Source: Transcode DNxHD .MXF to ProRes for FCP editing