Flip-flop freewheel to fixed?

Is it possible to change the freewheel side of the flip-flop hub on my Langster to fixed, i.e. so that I have two fixed ratios to choose from?

No.

Brief, but to the point - thanks!

The long answer would be that the freewheel side does not have the additional reverse threaded bit and so you could not install the lockring.

However the short answer is just as useful.

you could weld the cog on

You're best just getting a brand new hub, though unless you're playing polo or racing track, having to fixed cogs doesn't really have that much of a use, you're most likely gonna use one for everyday riding and a lower one for when you're shattered, in which case you're better off with a freewheel. Only if you have a rear brake though.

So a sprocket will thread on okay, its just you can't lock-ring it on? So you could rotafix it on really tight and promise not to back-pedal...? (Audax events on fixies allow swapping between two fixed gears, but you're not allowed to swap between a fixed and a free - doesn't sound worth the hassle/risk though).

Yeah you can do that, it's called a suicide hub, i'm sure you can imagine why ;)

I knackered my lockring once, rode it home with only a front brake, kept backpedalling without thinking, unscrewing the locking and having to pedal really fast to screw it back on again, that was a fun experience.

So a sprocket will thread on okay, its just you can't lock-ring it on? So you could rotafix it on really tight and promise not to back-pedal...? (Audax events on fixies allow swapping between two fixed gears, but you're not allowed to swap between a fixed and a free - doesn't sound worth the hassle/risk though).

No point really. You could rotafix it on really really really really tight and it is still going to come loose if you apply back pressure to the pedals. New hub+spokes+build is not very much money, especially if you build it yourself or ask someone from here to do it.