What I've been trying to do is take pieces from one armor and add it onto another, or have it take the place of the original piece. When I finish placing them and whatever, I go into the GECK and bring up the files but the pieces I've edited don't show up. If I've replaced one armor's legs with another's, then there are no legs. The only way I can get editing stuff to work is if I only remove pieces, but I'm trying to add them. What do I have to do so that those added parts show up in the GECK and the game?

Sure you saved everything in the right place/format?

And I'm not sure about this but there could be a budget on armor. Too many textures or pieces and the GECK is forced to cut something out. Doubt it, but who knows.

Make sure the pieces you're adding, you do Copy Branch from the mesh you're copying from, and a Paste Branch into the Scene Root of the one you're copying to.

As in, Right Click -> Block -> Copy Branch
go to destination NIF
Right-click Scene Root -> Block -> Paste Branch

Ah, thanks. That got it working

No prob, it's an easy mistake to make, I know I made the same mistake at first.

OH I forgot to mention, you usually have to rename the block once you paste it, I've had odd effects from not doing so (it took the name of one of the bones, which caused some weird issues with the pip-boy). If you give it the same name as it had in the source NIF that should be fine.