This is a great starter project
And you do not need to convert the teen to adult, you can use the adult nude top and my default replacement.
You can open both meshpackages with Simpe, right click the gmdc and save it in a folder.
Those 2 meshes(gmdc) you can open in Milkshape
-import the bottom first
-import the top(do NOT import additional bones!! (Do import the morph's if you want one)
-Go to the groups tab and select both the top and bottom, then regroup them together
-Rename it to body
-Go to the comment box and delete the lines(i think the last 5) of the top and change the name ''bottom'' into ''body'' in the 1st line
-Hide the new fullbodymesh
-Copy the morphname of the bottom
-Select both the bottom fatmorph and the top fatmorph, then regroup them together
-Rename it back to the correct morphname(right click and past)
Do the same for the pregnant morph
export your file.
This is quickly explained, prolly forgetting things but there are tons of tutorials that do it step by step.
hmmm, reading your post in the request section there is a easier way...
Ill save you the trouble of making a package and using Milkshape
*Open the meshpackage from the milla fullbody with blockfeet with Simpe
*Right click the gmdc, choose replace
*browse to the afbodynakedHI-34ax34_MillaJ and import it(downloadlink below)
*click commit
*Save