Hi,
I don't know about extending the existing cable, but it might just be simpler to get a longer one instead.
There were some PCs and Mac clones which used multi-device cables, and those are pretty long in some machines.
You could also get one out of a Quadra 900/950, or perhaps a Mac II.
I hope this helps. I don't really have any other ideas, unfortunately.
Hopefully someone else will chime in.
c