if this question is focused only on using a freehub as a rear hub on a single speed the answer is yes, just remove the cassette, buy a single cog, making sure it is compatible with the splines on the hub, and get enough spacers fill in the rest of the space on the freehub.

it can become more complicated though, once a preference for gearing becomes an issue.

i have a 97 Gary Fisher that i converted to SS as described above, and was lucky to find that the 36 tooth chainring and the 17t cog (both part of the original componentry) would, after removing a few links, give me a decent chain tension. as mentioned "luck" was on my side. there are a few tricks (see sheldon's site) that can provide a few more gearing options without resorting to a chain tensioner.

as a first step, i might break the chain, bypass the rear derailleur and try to find, by trial and error, a combination of chainring and cog that gives a decent chain tension. then re-install the chain with the unnecessary links removed and run it that way for a while and see how it goes. poor chainline could well be an issue at this stage, but it could give you an idea of what's possible with what you've got already.