You need a new hub/wheel. The thread is different. For fixed it's a "stepped" thread with two different sizes for the cog and lockring, but for a freewheel it's one longer thread.
I thought you could thread enough of a freewheel on a fixed hub using the cog's threads. Am I wrong? I'm sure I've read it on here somewhere.
Sorry just refreshed the page and noted the above comments.
@Cranky started
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You need a new hub/wheel. The thread is different. For fixed it's a "stepped" thread with two different sizes for the cog and lockring, but for a freewheel it's one longer thread.