I know there's a Mac keyboard out there that is supposed to feel like the ADB Extended II. I also believe the company that now makes the Model M makes a USB version--if you like the ADB Extended you'll probably also like this one, it's the one that IBM used to make (until they sold the design to Lexmark and then this company, I think it's called Unicomp).

There are adapters, yes, that can go from ADB-USB and PS/2-USB. I'd check compatibility with whatever hardware you're using, I'm assuming you're upgrading to OS X?