you can thread a singlespeed freewheel onto a hub that was designed for a multi-speed freewheel. however, the cog of the freewheel will be difficult to line up with the front chainring. (I'm guessing you know this and that's why you asked.) you can put just a few millimeters of spacers behind the freewheel, but I would not think it safe to put more than 5mm because there are not enough thread on the freewheel after that. that's most likely not going to be enough to get the chainring and freewheel to line up.
you can try moving the chainring in by using a narrower BB spindle and/or moving the chainring inside the spider if it's not already there. the limiting factor will be the point at which the chainring makes contact with the chainstay. again, that's only going to get you so far in your quest for an adequate chainline.
lastly, you can probably re-space the axle. if this is a conventional cup-and-cone hub, there will be a cone nut, wide spacer, maybe some washers, and a lock nut on the right side of the hub. you can remove the axle and rearrange those spacers all you want until the freewheel is closer to the drive side dropout. just make sure the overall spacing is the same when you're done so the axle fits in the frame correctly.
the next problem you need to tackle after the axle re-spacing is the rim dish. effectively moving the hub laterally on the axle means the rim will come with it and the tire will be off-center in the frame—maybe even bad enough that the tire will rub the stays. you MIGHT be able to mitigate this by adjusting spoke tension until the dish is correct. however, this will likely mean that some of the spokes will be engaged all the way in with spoke ends sticking out into the rim, and the other side with nipples that are barely engaged on the spoke threads at all and likely to strip out and or break. ideally, this means the best way is to completely unlace the wheel and rebuild it with the spokes on the opposite side from where they started: longer spokes on the drive side and shorter ones on the NDS. if you have any skill with wheel building and the wheel is in decent shape, you can do this in less than an hour for $0. I've done it several times.
if that sounds very complicated, you can find a singlespeed wheel with a thread-on freewheel hub pretty cheap.