I need to add another bone to an already deformed armature model, but it won't let me. Is there something I'm missing or is there any way to form back the armature and then deform it again, but this time with the extra bone included? I really don't want to have to rig the entire thing again.
Go into 'Edit Mode', Shift+a to add an armature bone. Parent the new bone to the existing armature. Place the bone within the mesh at the proper location. Even if the mesh model is already parented to the armature, parent the mesh again to the full new armature.
In 'Edit Mode' the PoseBone deformations will not show. In 'Pose Mode' you can deform the armature and the mesh model will follow.
If the automatic weight painting is not satisfying, you can manually edit the weight painting for each bone in the armature. You do this by clicking on the mesh, and then selecting 'Weight Paint'. Then select the armature that you want to fix the weight distribution.
First, go to edit mode after selecting the armature.
Then, extrude the bone which you want, go to pose mode and shift-select the armature.
Then go to weight paint mode, ctrl-click the bone you want to add new weight.
Finally, left click to mesh it will create new vertex group for the new armature.