Combine parts in outfit studio

[ModEnjoyer52]

I have downloaded outfit called [Melodic] Selected outfit SE 3BA.esp (nsfw blog). I am making it SFW. I removed the lewd alternate versions in xEdit, but when I equip only one of the two pieces there is nudity on the other half of the body. Many outfits are like this that are otherwise good for my game. Is there a way to fuse the top and pants to create a single item? Or another way to keep underwear on the non-clothed half of the body?

[scorrp10]

Yes, you definitely can  combine them in Outfit Studio.  First load the top, then in File menu, use "Add Project" to load the bottom part.  Then in the mesh section, for each mesh that belongs to pants, go to Partitions tab, and change their partitions to Body (32), then save it, open it in Bodyslide (might need to restart Bodyslide) and build.

Another option is to do it via CK.   Load up the mod, go to its Armor section, find the pants part, and check which ArmorAddon it uses and which slot it goes into.

Then locate the top armor form, and in its list of addons, add the pants addon, and in the slots list, add the slot for the pants.

[ModEnjoyer52]

Thank you, I managed to follow your instructions in outfit studio. In game all color variations of the outfit appear as the one in the project (black). For this I am confused.

[scorrp10]

I see, so that armor set includes color variants via texture sets.  

I take it you  decided to use my first method (combining meshes in outfit studio)   Unfortunately, this can easily play havoc with alternate texture set assignment order in CK.   Here is a primer:
Assume you have a file Top.nif   containing two meshes:
top
CBBE   
Internally, top mesh is assigned black textures.

In CK, in the ArmorAddon definitions, you have one called TopBlackAA, which points at Top.nif, and assigns no alternate texture sets, so it is black.

But you also have TopWhiteAA, which points at the same Top.nif, but if you click Edit button, you can see it has an alt texture assignment:
CBBE
top             TopWhiteTS
Where TopWhiteTS is a TextureSet, and also note that in CK, the order of meshes is reversed.  But texture sets are assigned to an ordered slot.   So, important part - TopWhiteTS is in the 2nd slot.

If you edit Top.nif in outfit studio, and add  mesh called pants, the list now looks like this:
top
CBBE
pants

If you now try opening that ArmorAddon in CK, you will see this:
pants
CBBE       TopWhiteTS
top
TopWhiteTS is still applied to 2nd slot, which is now CBBE.  And CBBE is the body, so it uses Skin shader, and its texture assignment will be overridden but the Skin TextureSet used by the body of the wearer. While top and pants have no alt texture set assigned.

So, your options are:
1.   Use the 2nd option from my original response (that is, combine top and pants ArmorAddons in same Armor form) or
2.   Got through all the 'variation' ArmorAddons and correct their alt TextureSet assignments, so it looks like:
pants        PantsWhiteTS
CBBE
top            TopWhiteTS
(You would need to look at a pants ArmorAddon, see which alt texture sets they use, and assign accordingly)