Uhm, don’t worry about that 
MonoGame should be able to import .x files, there’s just no runtime. You can write your own or check out the discussion on the Skinned Sample and Better Skinned Sample: Tutorial: How to get XNA's SkinnedSample working with MonoGame - #66 by Milun
Source is included in the samples, so you can pop it right into your project if you want