I need to mount a 13mm drill chuck onto a stepper motor shaft (don't ask!)

It seems most 13mm drill chucks come with a 1/2" female thread. Therefore I need a stepper motor shaft (M8) to 'drill chuck' thread widget/adapter ....obviously no such beast exists.

Now then, I could lay out for a 1/2" 24tpi UNF die ...but this is a one off that I'll never use again...so that not very cost effective. Therefore I was wondering ...would it be do-able to just re-tap the drill chuck's existing female 1/2" 24tpi UNF thread...to be an M12? (because I already have metric taps/dies)

Or will this be problematic?! (one issue jumps out, is that 12mm is slightly smaller than 1/2")

(btw the end result - if I get there - will be under light load)