I imported this model from the Asset Store. I clicked on rig → humanoid → create from this model ->configure. Then I went to Mixamo site clicked FBX for unity → import → ‘anim’ rig → humanoid → create from this model. If I would drag the avatar I already created with my character I would get a miss-match error with the hips. I don’t know if I can solve this due to the fact that my model has only 1 bone in his chest area. Is there a way how u can add bones maybe? Anyways this problem didn’t allow me to get the exact anim from Mixamo. Is there a solution?
to completeShdi answer, you can export your model in .fbx 2013 version 
(FBX keep different parts if there is more than one)
Simple answer is no. Reason being that you cannot just put a pre-skeletoned model into maximo and then import into unity. What is actually happening is that the pre-skeletoned model actually contains two sets of skeletons.
Possible Solution:
Remove the skeleton from the model in 3D’s Max or Blender, export the model as an obj. Then import into mixamo doing the same thing you did earlier and import into unity the same way. This should get rid of the issue.