You can not add more disk to a existing disk group if there are more than one virtual disk on that group. 

Below link have more details on that

https://www.dell.com/support/manuals/en-in/poweredge-rc-h330/perc9ugpublication/reconfiguration-of-virtual-disks?guid=guid-aad79692-439f-4743-bfbe-1f054957cd49&lang=en-us 

"Online Expansion Capacity is allowed on a disk group with a single virtual disk that begins at the start of the physical disk. It is not allowed when there is a free space at the beginning of a disk."

You can not merge two virtual disk to one as well.

I would recommend to take backup of both virtual disk and delete existing virtual disks. Noe create new virtual disks with all 5 drives and copy the backup to new virtual disks

Another option is

• take a backup of second virtual disk you created
• delete the second virtual disk
• add more disk to the backplane and expand the existing RAID to add these 2 drive and expand the capacity of first virtual disk
• Create one more virtual disk with required capacity

This second option will only work with only single virtual disk in RAID that begins at the start of the physical disk

You can create a third VD with the new 2 hard drive. We can not add those drives to existing VD as there are 2 virtual disk part of that group 

So I cannot add a third VD?

You can add a third VD, but it will need to be a RAID 1 with two disks.

Only way to add them to your existing RAID 5 is to backup and delete the second VD, reconfigure the RAID 5 to add the new disks, recreate the second VD and restore the data to it.

Ok, thanks.