Adding extra HDD’s to R810

I currently have an R810 Poweredge and want to add some additional storage. The configuration is listed below. 

• 4x Intel Xeon E7-4870 2.40GHz 10 Core
• 128GB - PC3-10600R DDR3 Registered
• PERC H700 RAID Controller w/ 512MB Cache
• 2x 750W PSU for R510/R810/R910
• 6x Enterprise 600GB 10k 2.5" SAS Hard Drive

I looking to add at least another 10 TB of storage for the server in the near term and it would be great to have the option to expand beyond that at some point. 

What are the best options to expand the storage of the system?

After several hours of googling it looks like the only option would be an external hard drive enclosure with an additional internal raid controller. Since compatibility will be an issue can anyone recommend a setup that would work well? Or if someone has a better solution then I would be glad to hear it. 

Also, to offer some background, the use case is going to be a server running Unraid. It will have a few VM’s and docker’s in it.