Its doable, however you would need Profession Services and Support to help you out on this task.
Is your data / storage pool simply using allocations from the DAS, or are you mirroring to the DAS (you asked about a mirror near the end).
Yes - you will need to power off the ELM to remove / attach the DAS units, as they are SCSI attached, and when you attach the second DAS to the one ELM, you will need a second SCSI card (which should come with every DAS).
Note - you should power up the DAS first, and then the ELM, or the system may not recognize the storage if done in reverse.
You could open a ticket with support to request assistance.
In theory you could setup the DAS-50 to be a mirror of the current DAS-25, once the data is replicated, fail-over to the DAS-50, break the mirror, then move the DAS-25 to the other ELM, of course you would have to wipe the data from it to regain the space.