Installing a rear derailleur on a bike with a nexus 8 internal hub
This isn't going to be nice, because the frame doesn't have a  derailleur hanger built in, or routing for the derailleur (which you'd have to hack on with clamp cable stops or something). 

On a normal frame, one would install either a derailleur with a built in claw (e.g. Shimano Tourney RD8153)



or an adapter claw (e.g. SunRace Rear Derailleur Adaptor Claw)

 .

(Pictures from Sheldon Brown's site)

But your bike doesn't have horizontal dropouts -- it has forkends. So, you could try a chain tug with a derailleur hanger on it:


This one is from Problem Solvers and is about 30 USD, but there are others from SunXCD, Origin8 among others. Then, bolt on a derailleur as usual.



Note that even if you get a derailleur on using the chain tug with derailleur hanger, you still need:


Wheel
Derailleur
Cassette/freewheel
Shifters
Cable routing