Dear John,
you mean an external device that can interact with GNU Radio's sample
streams?
Well, I'd say it's not very hard per se – all you need is to write a
driver for that device. That driver needs to have an interface that you
can use to transmit samples from your PC to the device, or get samples
from that device into your PC.
Then, you'd call that from within a GNU Radio block. It's pretty much
exactly what the USRP sink and source, or the audio sink and source do.
The question of "how hard is it" is, however, tends to be dominated by
boundary conditions: How fast do you need things to be? How experienced
are you at building devices, and how experienced at writing drivers? Or
do driver and device already exist, and you only need to write that GNU
Radio block? Then it's mostly a question of programming proficiency.
Best regards,
Marcus