Presumably it isn't ALREADY a USB + PS/2 combo mouse - that's a very common style where the mouse has a USB connector, and includes a PS/2 adapter - this is NOT a universal adapter!
Another old style, was the PS/2 - Serial Combo - If I remember rightly, the mouse had a PS/2 connector, and included a serial adapter (again, NOT universal) - if it had one of those, find it and transfer to serial port.
Keyboards are a bigger problem - unless there is BIOS support for a USB keyboard, it would not be possible check/fix BIOS settings - the BIOS generally doesn't care about a mouse, though there was the brief aberration of mouse control in some 486 BIOSes.
PS. I've been running a fully optical mouse for ages now (corded, and now a cordless with charging holster) - I'd NEVER go back to a ball.
I've also used tracker balls (they get VERY dirty) - and when you get used to the inherent 2-speed operation (holding, or flick and spin), you get very quick.
Favourite device was a BTC keyboard with glidepad, but the keytop lettering wore out (and I'm no touch typist!)