Then I would not put them in the same pool.
That new member is a hybrid designed for low latency. That you are stripping to a slower array with more capacity.
You can always merge them later, but doing the opposite is a more involved, longer process and you must have enough free space on the larger member to store the data from the new one.
You can move volumes from the 7200RPM array to the hybrid live w/o downtime.
Make sure you switch has enough inter switch bandwidth to support the additional array.