i see Vash is doing this, but i thought i'd stick up the solution anyway, as editing an existing model can be one of the easier things to do with milkshape.
tools > half-life > decompile normal hl mdl model
you are presented with a choice of which parts of the model to decompile ( reference, sequence, texture and .qc file). check all of them. hit ok, and all the files you need will be created in the same folder as the original model.
as the reference file (the one you will want to edit) has the same .smd extension as the sequences, you will need to find its name before you can import it to milkshape. you can find its name in the .qc file. open this file with a text editor, and look for something like this
$bodygroup body
{
studio "rocket4"
}
in my example, rocket4 would be the name of the smd reference file. so in milkshape you should now go to file>import>half-life smd. select the smd that was named as the bodygroup, and edit away.
when your tampering is complete, export the smd back to where it came from. (with the same name of course) when you are asked if it has to be exported as a sequence or a reference, choose reference.
now, in ms again, go to tools>half-life>compile qc file. select the qc file, hit ok.
if it has all gone to plan, a spinky new model will have replaced the original.
oh, remember keep a copy of the original model in a safe place before you do this.