Make sure you've got a child body (for reference) and your resized adult body in MilkShape. Make sure that, in the Groups tab, your child reference mesh is on top. Then you need to select one neck vertex on both the child and resized meshes (they're probably in exactly the same place already, so you'll probably select both of them by default when you click on them). Then do "Vertex Normal Merge". Work your way around the neck doing this. When you're done, delete the child mesh you used as a reference. Save, export, etc.

it is a bit tricky on the collar but doable.
To setup the merge tools you can have a look at this tutorial:
http://www.modthesims.info/showthread.php?t=398865

Oh no!

I tried to do it but the results aren't "nice"
I'm always catching issues...

And his neck isn't fixed at all...

Oh, dear. One of those meshes. I was hoping that the neck would actually be separate from the collar all the way around, but it looks like it's not. That's why you've got weirdness on the white part now. Hmmm...

Let me have a quick look in MilkShape and see if I can offer a better suggestion for you.

EDIT:

Okay, I had a look. Your necks aren't lined up quite right, so you're going to have issues because of that. I think what you need to do is select the two vertices (one from the modified mesh, one from the child mesh) like you did before... but first do "Vertex Data Merge". Do that all the way around. It looks like the only vertices you need to also do "Vertex Normal Merge" on are the three along the front of the neck (the one right in the middle and the ones on either side... before it turns into the collar). I haven't checked what it looks like in Body Shop, but the neck line is probably gone and there shouldn't be weird shadows on the collar now.

Hi,

I could only fix one issue...unfortunately :/
There aren't weird shadows anymore
But his neck...

Edit: I moved the wrong vertices to correct it... I don't see a difference between the child and my rescaled body now...then I did "Vertex Normal Merge"
If I do "Vertex Data Merge" the neck looks really strange in MilkShape and I think it would looks stranger in Bodyshop...

"Vertex Data Merge" should not make the neck look strange at all; it didn't look strange when I did it. All that tool does is copy the vertex information from one vertex to the other (that information includes the exact position of the vertex and the bone assignments). If you just eyeball it until it looks right, or if you do "Vertex Normal Merge" without doing "Vertex Data Merge" first, you could end up with gaps and lines when the Sim is in the game. I can sort of already see that happening with the picture of the back of the collar.

If "Vertex Data Merge" is distorting your collar, you're probably selecting too many vertices. You just need to select the one on the inside edge of the collar (plus the one from the child-sized reference mesh, of course). This collar has thickness, so it's easy to accidentally select two vertices instead of one (which will "flatten" the collar against the neck and make it look different from the original).

you snapped the outer vertex from the collar with it, that is why it looks strange?

You best go to side view and select the top row of vertice.
Then choose select inverse from the menu.
Choose hide selection from the menu so only the top row is visible
Select those vertice and go to top view
right click in your window and choose frame selection.
now you can easily select the inner ones from the neck and collar and use the data merge tool

the front skin part is a bit tricky.
i ungrouped those few faces temperary so i could fix the normals.

The neck is fixed now

Thank you BloomsBase for the fixed mesh
And thank you fakepeeps7 for the helpful advices

Without you two, I couldn't finish this converting


And I'm not sure about it...
I don't find this outfit here in MTS...
Should I upload it now?

You can upload it at some point... but right now the uploading system is offline for "spring cleaning". There will probably be a message on the main page when it's ready to go again.

Oh, thanks for telling me that I can't upload it at the moment.

I finally upload it

Here