az2008

Yes. If you have a good rim and the number of spoke holes match the spoke holes of the hub you buy, all you need is new spokes. Lacing a wheel is a bit of a project. It's just a lot of minutia you have to take into account, and a lot of faith that the spoke calculator is correct. I did mine 6-8 months ago. I might have saved $50 over my LBS doing it for me. But, I put a lot of time into it trying to understand it all. I learned a lot. But, I had a lot of time on my hands. Not everyone will be that ambitious. This is one of those things that would be better left to a shop unless you have a lot of time on your hands and/or you're really smitten by cycling and *want* to learn (experience) building a wheel.

I did this Nexus 7 hub, this Sun Rhyno Lite rim, and my LBS has a spoke cutting machine. I went there with my exact spoke lengths[1] and paid about 75 cents per spoke ($30). (There's a slightly narrower Sun CR18 rim if you intend to run narrower tires. I chose the wider rim because eventually I want to put some fatter 2" tires on the bike. When searching for 20" rims you also have to be cognizant of the fact that there are two standards. The popular 406 diameter and a less used 456 which is narrower and more like a mini road-bike wheel. Sometimes referred to as 1-3/8".).

Since you don't have a derailer you'll need to do something for cable guides. I'm not familiar with what's available. My bike had one continuous cable housing which passed through 4-5 brazed on guides. It didn't have the more customary guides where the cable housing stops and bare wire runs to the next one. For a folder, continuous housing might be better since it allows the cable to float and adjust itself to the bike being folded. (Point being, you could get by with loose zip ties and continuous housing.).

I still need to post photos of my finished product to this thread. I intended to give it awhile before cutting off the v-brake posts (make sure I'd never want to go back to the original configuration).

[1] I used spocalc and this and this site's explanations of the measurements.