Uploading/Editing MD Avatar Parts!

Hi everyone! I've been googling, searching, rummaging for a couple hours and have found no answer, not even on reddit or MD forums, so this is my last resort. I'm trying to add an "appendage" to my MD sims 4 male avatar to create more realistic pant-looks. I cannot, for the life of me, figure out how to do it. The farthest I got was creating the mesh in Blender 2.79, exporting it as a .obj, and uploading it as an avatar in MD9, which works well except the shorts on the "nude" male mesh appear in MD9. 

My question is: How can I add a "member" or slight bulge mesh to my sims 4 male avatar without including the default nude shorts mesh from EA? I truly thank you!!