Floppy Arms - Deformer/Armature Issue

[Khreme]

So I modeled an avatar and would be in the last stages of finalizing it; however, its arms weren't quite in a t-pose during modeling, so this came back to bite me in the end--the arms cross into the body in sl. I tried to edit the armature and mesh in blender but I couldn't get the t-pose without a modifier messing up or the mesh unbinding from the armature (And even if I did, the angle would likely still be off). So I decided to just do an arm deformer.

But the method I found is outdated. Literally every avatar I own comes with a deformer but I have no clue how they are made--the ones I rezzed are just invisible, empty mesh boxes. I expected them to contain an animation or something.

Could someone please point me in the right direction? 

(I know some people don't like deformers, etc. because they don't use them, so I would like to ask that responses please be limited to things that will answer my question or maybe tell me how I can fix the arms in Blender. I've looked in other threads and haven't found even the slightest hint because of this.)

Thanks in advance.

UPDATE: I have resolved my arm issue, but I want to leave this up just because this deformer question is very relevant since I couldn't find anything on them and they are obviously still being made. So any responses will be appreciated, and hopefully helpful in the future!