Sure, just set the jumpers on the HD ion Slave position (you might even be able to find one of the original blue blank covers to replace the slotted one.
Here is the status: I found a better (in my sense) solution:
I bought a cheap Seagate 40GB 7200 HD ($66) and a flat cable supporting 2 IDE drives... and I installed it on the UltraATA driver. Only important thing: make the new drive slave (no jumpers).
I had some more problems with porting the 5 users to the new drive, but finaly it was ok. I just recreated the account after having installed MacOSX on the new drive... and I created these in the same order as on the old 6GB drive.