if i get you right, you have already smooth skinned your model. and you probably have a few blendshapes already as well. right? now you need to add more blendshapes.
for that, here's the solution. you select the both the models in proper order, then you go to deform>create blendshape>options. there, under the advanced tab change the deformation order to front of chain.
that should solve your problem i think.