Upgrade a KVM switch to USB
Well, with a lot of luck this might work.

But really, that's only luck: USB does actually require some bounded impedance changes on its lines, and while just "any" through connection would work for USB low speed / full speed, you really can't expect things to be reliable, or faster USB drives (USB2 HiSpeed) to work.

Anyway, product page says:


  Keyboard Caps Lock, Num Lock, Scroll Lock states are automatically saved and restored when switching among computers.


This means the thing "speaks" keyboard on its DIN port, and hence, won't deal well with USB.

The product page further suggest you can cascade it three levels deep – it wouldn't surprise me if there was slight amplification on the PS/2 port, too, so, that wouldn't be usable for USB passthrough, either.

Ah, even moreso, it speaks of "keyboard emulation" and "mouse emulation".
This used to be a nice device, which "played" mouse and keyboard. It's not just a passthrough. Your plan probably won't work.

Let's face it: something for serial mice was simply useful in the early to mid nineties. Something with a DIN connector for a keyboard was useful in the late 1980s.

I'm personally happy the VGA days are gone, too. Modern LCDs are simply way, way crispier than even the midrange professional screens I used in that era.

I wouldn't go that route, but shell out 13€ or so for a USB switch. I just couldn't stand the excitement whether my mouse works reliable or not. You shouldn't, either.