If it is a partial profile insert and the UN thread lies within the metric range it is made to cover a functional UN thread can certainly be produced. It would be advisable to draw things out so as to select appropriate infeeds and starting dimensions for close to maximum metal conditions which should give a respectable approximation to proper UN thread classes. You may have to accept close to the proper class size as being as good as you can get.
Not something to do on a customer job!
If the metric and UN pitches are sufficiently close it is possible to produce a thread that screws on and holds but engagement percentage will be lower than ideal. Frankly a temporary bodge to get things going pending sourcing of the proper tool.