You could create a RAIDZ2 VDEV in a degraded state without two drives, migrate the files, then add the drives… but it’s both risky and a PIA.
Suggested readings: iX's ZFS Pool Layout White Paper and Assessing the Potential for Data Loss.
Re: RAIDZ2 espansion, be mindful it will have its own cost as well. It’s no magic hand. Do note the caveats.