Hi, I'm trying to add completely new hair to The Sims 3 (and in that, I mean it wasn't created using any of the games original hair meshes), but I can't get it to stick on the Sims head like normal hair. Instead it just floats around in odd positions when the Sim moves around. Is there any way to fix this? I'd really like for this to work.
Also I'm using TSR Workshop, if that helps any.

Even without looking at the details you neglected to mention, it is evident you have not made any assignments of the hair to the head (perhaps it is the head_new). Look at joint assignments for one of the original hair meshes and then assign your vertices to that joint, too. Assignments are how the game knows what joint to follow when animations run.