REMEMBER TO BACKUP BEFORE MESSING WITH DISKS OR PARTITIONS/VOLUMES
Tested on DSM 6.1 through ESXI 6 here.
Few days ago, my DSM started complaining about low disk space available on volume 1.
It was only 16gb.
So, i tried to expand it few times after expanding .vmdk but "Extend" buttond stay grayed.
After few tries and installations f*cked up, here is the way i did it, and it should work on barebone install too :
Keep the disk you want to "copy" in the Diskstation, then put the new drive in avalible slot, and make them to RAID1 - mirror.
When the its done, then remove old volume 1 disk and add change freshly added disk to whatever was ancient volume 1 (i.e scsi 0:0, or 0:1).
This will give you a degraded state Raid 1 with only new volume 1.
Then activate Telnet an use this command :
mdadm --grow --raid-devices=1 --force /dev/md2
(RAID1 to BASIC)
Now the new disk is basic again an you can expand it to its full size.