If you end up getting a 1/8" cog or freewheel you can still keep your 3/32" chain ring, you just have to run a 1/8" chain on it.
Not sure what you mean by your second question, but if you're thinking about putting the freewheel shell around a fixed gear cog then that won't work. Choose one or the other. I bet your Origin8 hubs probably have the ability to be flipped so you can put a fixed cog (don't forget the lockring too) on one side, and the freewheel on the other if you want to try both.