Do you mean FlashPool? If you have a system with say.. 4 SDDs and 20 HDDs. It will look like this.
The 4 SSDs will be left alone and the 20 HDDs will be RD partitioned. With the 4 SSDs you have two options. FlashPool, just adding SSDs to an aggr. Not the best use for a system this small. Yon can opt to create a StoragePool that will have 4 even partitions and you can assign the partitions to each aggr.
No. You can't really convert into with out doing a re-init.