Actually, stopping any in progress Scrub and preventing new Scrubs from starting should be standard for RAID-Zx expansions. After all, immediately after the expansion is completed, a Scrub is performed because the data re-columned did not have its checksums verified.
Now the stopping of in progress Scrubs on pools attempting to be RAID-Zx expanded, probably should be a prompt in the GUI. Like:
There is a Scrub in progress, starting RAID-Zx expansion would stop it.
Okay?
And of course, it would be only for a pool being expanded. Unrelated pools can start and continue their Scrubs without interference.
Edit: Looks like a Feature Request to stop Scrubs during the expansion is already part of this request: