What you do is open both files in a text editor. In the V3 file, scroll past all of the actor Whatever { geometry stuff} entries, until you get to the second actor Body, which has bunch of lines and then a "channels" section. In the channels section, you'll find channels that start with "valueParm PBMStretchArms" or legs or neck or whatever. Copy the channel, which is the valueParm line, the following {, and everything up to and including the matching }. Just a normal text edit->copy operation. Now, switch to the clothing file, scroll down to the "actor Body" section with channels, and inside that Channels section, paste the "valueParm" channel you copied. Save the clothing item (I usually save under a new name in case I messed something up.)

Now, open up Poser, load up the clothing file you just saved, and apply the "ERC Inj" pose (or whatever I called it. Something like that) from the happy magic zip I linked to up there. Spin the Stretch Arms/Legs/Neck/whatever dial and make sure things are stretching. From Poser, save it back to your library, and it's ready to use.