The threading on normal BMX freewheels and fixed gear cogs are the same. Your BMX hub probably uses the 1.37" x 24 tp standard threading. You can thread a fixed gear cog on that, but you won't have a lock ring to keep it from spinning off if you try to skid. You might not want that as a permanent solution, but it isn't unreasonable as an experiment to see if you like fixed gear.

The spacing on your BMX hub is probably 110mm. The road bike you want to convert probably has rear dropout spacing wider than that. Is the axle on your hub wide enough to fit? If so, you could add spacers.

I think you would be better off looking for a cheap pre-built wheel with a flip-flop hub.