Very interesting question, and one I've had to deal with directly myself.

We've got a bunch of those USB-to-PS2 adapters lying around because of all the USB mice we've bought and we don't need those adapters. But it doesn't do me any good because...

Now, this may depend on the KVM switch, but mine will flatly NOT work with my USB mouse plugged into a USB-to-PS2 adapter. The only kind of mouse that works in my KVM switch is a genuine PS2 mouse.

So here's what I do.

I have my nice USB optical mouse plugged directly into my main work PC.

Then, I have an old PS2 mouse plugged into the KVM switch. I've got one monitor, one keyboard, but two mice.

Either mouse controls my main work PC (the USB plugged directly, the PS2 via the KVM switch). But only the PS2 mouse controls other PCs on the KVM switch. When I work on the "other" PC's, I move the old PS2 mouse onto my mouse pad and I can switch back and forth between computers seamlessly.

When I'm done working with the "other" PC's and I'm back to being on my main work machine, I move my "good" USB mouse back onto my mouse pad for its additional comfort and accuracy.

Your other option is to replace the KVM switch with a newer one that supports USB.