many mouses are usb+ps2 compatible
here is pinout adaptator :http://pinouts.ru/InputCables/usb_ps2_mouse_pinout.shtml
Of course that's not an universal adaptator allowing any
or mouse connecting to USB !! This has to be done.....
Maybe with 'low end' AVR with this starting point :http://www.obdev.at/products/avrusb/projects.html
Maybe with PIC18F2550 using samples for USB/HID given with PicBasicPro, ProtonPDS, CCS, MCC18, Mikro..