I need converter from modern USB keyboard to old PS2 as some old computers does not support USB
and modern USB keyboard does not work with simple converters (already tested).
So I see a need for "advanced active" USB keyboards to PS2 converter.
USB is made of Hosts and Devices. The Leonardo can natively act as a USB Device. In order to communicate with a keyboard, it you need to emulate a USB Host.
The USB Host Shield has code examples to work with Keyboards / HID devices. You don't necessarily need a Leonardo.