Bicycle Mechanics - can i convert a 21 speed rim in to a singlespeed

hey i just ordered a motobecane messenger and i know the wheels arent going to be any good...i have a set of mavic askiums from my other bike which im no longer going to use and i want to know if it is at all possible to convert the mavic into a single or fixed gear...any help would be awesome

Converting a freehub-equipped wheel to a SS is quite easy, simply rip off the cassette and replace it with a single sprocket and a set of spacers.
But will your frame accept a 130 mm OLD hub?

It will work great. Stripping your cassette to only one cog is purely aesthetic.

It will work great. Stripping your cassette to only one cog is purely aesthetic.

Actually stripping the cassette to just one cog allows you to use the cog of your choice and get a perfect chainline.

The one cog approach sheds some weight but I do know a few folks who have set up ss bikes and left the freewheel / cassette intact to allow for a second gearing choice.

I tear apart freewheels all the time. Clean, regrease, and reassemble. I put them on fixies that I sell. But I've rebuilt several to single speed. I have a good selection of cogs and bearings that I save for just this purpose.
I've found, though, that many freewheels use the smallest cog as sort of a locking mechanism (what a lockring does on a cassette). So I usually have to leave it on. I've ground the teeth off to make it less noticeable, but you could leave it as is and have two cogs to choose from. The rear lug adjustment should be enough to take up the chain slack if the two cogs aren't too far apart in size. That way, you have one gear for around town, and another for a weekend distance ride.