P.s I've added some photo's to help you understand.

In order to be usable, the obj mesh needs to have bone assignments and UV mapping. From the picture it also looks like you need to fix the normals and/or reverse the faces. Once that's done, you can merge the two groups - select both groups in the Groups tab, then click 'Regroup'. Make sure the group comments are the same as the nude top. Then renumber the combined mesh (using Wes Howe's Autorenum tool) and export as a .simgeom.

I haven't actually done this process, so can't guarantee it'll work. BloomsBase can tell you more.

It would be better to remove the part of the nude mesh that's under the bra, so you don't get clipping problems. And of course you'll have to make morphs. You can use Mesh Toolkit to do auto bone assignments and auto morphs using the nude mesh as reference in both cases. UV mapping from scratch is difficult though.

I've been working off and on to add an obj converter to Toolkit, but don't know when I'll be done.

In general for this kind of thing it's easier to find an EA mesh that's similar to what you want and modify it, or take pieces from two or more meshes and Frankenstein them.

Okie dokie Thank you =3

As Cmar explaine, uvmap the top before you regroup it.
Keep in mind that the custom parts need to be uvmapped in a specific area, neither should they interfear with the nude top parts
Use this template:
As you will see there is not much room for it on the map

Oh, dont clone the nude top as clothing outfit.(i made that mistake in the past)
It has a diffrent setup as clothing, use a bra top instead.(were you can load the nude top with your custom addon in)