Honestly i just spent about 2 weeks or so learning blender. Genuinely this avatar was more of a learning experience than anything. What worked for me was actually the fbx exporter for unity. make a fresh project and extract it. mochifitter is an angle i considered however making the actual profile seemed a bit intimidating, and id rather direct the effort to pick up blender. That and adding simple outfits that are already weighted to milltina works very well in blender + even more intricate fits are doable. Personally never did weight painting/ transferring the weights or merging armature as tyexo mentioned earlier in the thread (just didnt understand it at all ngl but i started looking into it xddd). And frankly my approach might be really stupid but it worked out, so heres the process i followed;
1: Import outfit to unity
2: find fbx folder and then the fbx, drag straight into blender
3: position and scale the outfit pieces to sorta match the shape.

From here its a bit haphazard but i'd start out using the elastic sculpt tool (i prefer it when the size unit is set to view, so zooming in and out increases / decreases the size of your brush). There is another tool i heard of that is supposedly really good for stuff like this but never got my hands on the info, as frankly there is really not a lot of guides out there that address stuff like this directly, however they do inadvertently teach some stuff thats useful in what we're trying to do. Start large, move the hips first, then work your way around to the back. usually need to move the spine / neck of the outfit quite a bit in to match the shape of the body. after 10 minutes you should have a pretty well fitting outfit for most of the body. The legs however can be tricky. its important that when you export the avatar as fbx you disable the MA script on upper_leg_L, lower_leg_L, upper_leg_R & lower_leg_R otherwise its hard to estimate the correct height to position shoes and socks to. however you could always just send it and stretch it to fit the extended version, i did it once and it worked out kinda well, but safest bet is just to apply the same scale adjuster component to the same armature parts of the outfit once its in unity.

Anyways once thats all done its time to export, select everything in the outfit object (not the very top object since it wont let you drop down the fbx in unity if you do. so usually all of the mesh, armature and pose) and export as fbx. Once in the export menu set the include to selected objects. on the transform make sure the apply scaling is set to fbx units scale and the forward/up direction is correct. custom properties can be on or off, i dont know really but if something dosent work it cant hurt to retry with it enabled.

I apologize for the rambling, but i really am not qualified to speak on stuff like this, a lot of what i said above can very well be accidental or just straight up incorrect. However you can at least use it as a baseline to work off of ^^
All in all some googling, AI and elbow-grease oughta sort it out.