Active adaptors do exist, but they tend to be expensive, and would be notably more complex to make work. I'm not sure off the top of my head exactly what would need to be done, but it's a lot more complex than the passive version.
If you have the space, it's probably much easier to get a QFR or similar and use its passive PS/2 adaptor than to try and hack an adaptor together. Likely, you would need to use a microcontroller to emulate a USB host device, translate the incoming signals into proper PS/2 interrupts, and send those interrupts via a PS/2 keyboard interface -- none of which is native behaviour for a teensy. This isn't my area of expertise by a long shot, so it's possible I'm off, but it's definitely not as easy as going in the other direction.
Using a TKL board like a QFR gives you significant space savings over a Model M. You could also check and see if any iteration of the Poker has PS/2 capability, although I don't believe it does.