Let's clarify terms

Single speed - can be fixed gear or single speed freewheel

Fixed gear - the rear cog is "fixed" on the hub. There is no freewheel mechanism. No coasting, no backpedalling. If the wheel is turning the pedals are turning.

If you want a single speed, just buy a single speed freewheel from Harris Cyclery or your LBS, remover your multi-speed freewheel, screw on the single speed freewheel, shorten the chain. As long as it is a freewheel you can still route the chain through the derailleur as long as you adjust the derailleur to stay in one place. Probably easier to take the der off and shorten the chain.

If you want a minimalist conversion to fixed gear order a track cog and a bottom bracket lockring. Screw on the track cog good and tight then screw on the lockring and tighten the two against each other as you do with cup and cone bearing locknuts. Use blue Loctite on both the cog and the lockring and LOCK THEM UP TIGHT. The stress of stopping with a fixed gear can be more than pedalling, and you can easily unscrew everything. Not good while riding.

Better is to get an inexpensive wheel with a flip flop hub, track cog, and track lockring (left-hand threads). One side is threaded normal for a track cog and left handed for the lockring. These will withstand the forces of stopping better. Quality Bicycle Products (QBP) sells a very nice one that you can get through your LBS for about $100.

If you are actually talking about trying to fix the freewheel pawls to make a multi gear freewheel fixed I wouldn't recommend it. If you are a knowledgable bicyle mechanic and a knowledgable welder you could probably figure it out. A single speed freewheel only costs about $20. Track cog and bb lockring about $30. Add another $100 for a proper wheel for the best solution.