Easy peasy:
Grab yourself a model decompiler/compiler. I recommend milkshape as it can do both.
Now, decompile the HL model you want to assign into a new folder, but just the reference SMD and texture files.
Now, do the same for the CS model in the same folder, this time, export everything EXCEPT for CS textures, you don't need them.
Now go to file --> import --> HL SMD file or something like that, and import the CS reference SMD file. Delete all the verticies on the model but not the skeleton, then import the HL reference file WITHOUT the skeleton. Rig the model you imported onto the existing skeleton, this part is the trickiest. I'm assuming you do know how to rig, so if you don't, google up a milkshape rigging tutorial, its not that difficult.
After you finish rigging, export the newly rigged HL character on CS skeleton over the existing CS reference SMD. Open up the QC file, and delete all the bounding box information, save and then click compile QC file, should work fine.