RE: Expand RAID-10 Space for T420 Server With PERC H310
You cannot expand the size of a nested RAID level (10/50/60). You will need to do a backup/restore or create a second RAID 10 for additional server storage.
Can I buy 3.5" SATA caddy trays and use standard SATA drives?
Define "standard". Don't use consumer/desktop/laptop/NAS drives. Enterprise drives at a minimum, Dell-certified if the data or services are critical.
Can I hot insert these drive in the server?
You should always insert/swap hot-swap drives "hot".