Expand capacity of raidz1 pool

[Anders Jensen-Waud]

Hello there
I have a 1 TB raidz1 pool on FreeBSD 11 (root on separate volume). I need more capacity and am looking to migrate to a mirror with 10 TB (min).
How do I migrate the raidz1 pool to a new pool?
Constraint: I only have capacity for 4 drives in the machine. 3 are currently occupied (root is on a small ssd). This will be important during the migration period.
Best
Anders