Martins solution should work. Sometimes though the Skin maps act up. If that happens you can always duplicate your mesh and bind that to all the joints you need. Then select the first mesh and then the entire second mesh and use the copy weights tools (I say entire because you can use this method in concentrated areas by selecting only vertices of the second mesh). I digress though, with both meshes selected use the copy weights tools and voila everything should be skinned perfectly the way you left it on the first mesh but the extra joint or joints are now ready to be weighted.
Both methods are pretty quick I like this method more because depending on the mesh, exporting the skin map may take awhile as Maya writes them into the image files. Also skin maps may have stray verts that aren’t skinned like they were originally. Easy to fix if it happens.