You'll need a number of things.... but it is possible. I have just done exactly that to the recumbent trike I purchased back in February.
Firstly you need a wheel with the hub in it. If buying new, you have a choice now between 32 and 36 holes for spokes on the Rohloff Hub and once that decision is made, you are stuck with it for life. We only had 32 available us but the strength side of life has not been an issue. You will also need to make the decision if you want the internet or external gear changer system for the hub. Mine is the external gear changer (known as the black box because it is a black box). You will also need to purchase the gear selector and cabling to go with the black box.
You will need to look at the drop outs and purchase the a suitable axel adaptor. I was able to talk with to the bike shop I was buying my trike through who talked with an ICE mechanic (manufacturer of my recumbent trike) and sort something out. There was a dedicated one to purchase.
As mentioned you will need a chain tensioner. On my trike I have used a rear derailleur and a short piece of cable. There is a dedicated Rohloff cable tensioner, but I didn't see the need to go to that expense when a long cage RD would work... Long cage in my case because I also have a front derailleur on my trike giving me 48 gears to play with. On my tourer I don't have a FD, just the standard 14 Rohloff gears which cover roughly 98% of the MTB bike range. My tourer also does not have a chain tensioner, it uses an eccentric bottom bracket which works amazingly well and lasts and lasts. Currently mine has done over 12,000 miles and is not showing any signs of wear.
So yes it is possible and it is not that difficult to do. Cost wise well my rear wheel alone is roughly £1,500 and that was purchased 4 years ago now. The additional bits needed to move it from one bike to the other trike... cost around £350. Its not cheap, its not light, but the moment you get back to using a Rohloff hub, you know it is worth it.