Yes it's not possible to expand any existing Virtual DIsk Group. Only option to increase space for any Pool is to add new VDG. Otherwise take data backup. Then delete all and re-create. Then restore from backup.
Coming SSD, yes you can create Read cache but that will help to improve read performance only. You should consider SSD use as seperate tier or Performance tier so that you can get benefit of both read and write operation.
Hope this helps!
Regards
Subhajit
I am an HPE employee
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