FAQ:I already have a 3d object from elsewhere. How can I use it in MH?
Short answer is that you can't, not directly. MH can only use assets which are made specifically for MH, in practice assets produced by MakeClothes.
If you have a 3d object you want to use (say, for example, clothes or a necklace), you have to use MakeClothes to convert it for MH first.
In this case you can import it in blender and use it for the clothes part in MakeClothes, as if you were modeling clothes. See the documentation on makeclothes for more information.