Synthesize ATPG Test vectors with 'X' Values
What you ask for makes no sense: A test vector is an actual signal. A signal doesn't take the value "don't care"; it takes one of two states.

It might not be in a well-defined state, but you can't produce "not well-defined" as state.

You probably just want to try a random subsampling of setting the Xs to either 0 or 1.