So from what I am reading, you have a MM-VA-500 server along with the licenses. You can verify the server config using the following command (output also shown).
(MM1) [mynode] #show inventory
Mgmt Port HW MAC Addr : 00:0C:29:xx:xx:xx
HW MAC Addr : 00:0C:29:xx:xx:xx
Product key# : MMFxxxxxx
Activate license : Not Applicable
Active device type : MM-VA-500
In this example, the VM build shows that this is a VA-500 MM.
From what I am reading, you need to add an additional 500 licenses and you want to be able to manage them from this MM. Buying the licenses is the easy part, however you MM-VA only supports 500 devices. So what you need to do is upgrade your VM configuration to make your MM-VA into a bigger MM, allowing it to support more devices. This is actually not as difficult as you may think.
The first step is to shut down the VMM. (of course take a snapshot and any backups). If something goes wrong, don't blame me. :-)
The VM has 2 disks. Disk 1 is probably 4GB and contains the bootloader, along with the two ArubaOS system partitions: System Partition 0 and System Partition 1. This acts as your recovery partition if you need to expand the size of your disk in the future, or if you perform a 'write erase all' from the CLI of your WMM. This disk should not be modified or deleted. Hard disk 2 is what you need to modify, along with the VM memory and CPU settings.
You need to go to the "ArubaOS 8.x.0.0 Virtual Appliance Installation Guide.pdf" and find the "Memory and CPU Allocation - Mobility Master Virtual Appliance" table for the MM (there are 2 tables one for MMs and one for MCs). From that table, identify which MM-VA you want to build. You can build a bigger MM-VA but install a lower number of licenses. For example you currently have a MM-VA-500. You could update your VM to a MM-VA-1K or even a MM-VA-5K but only install 1000 licenses. This is not a problem, it just uses up more VM resources. So in your case, you probably want to go to a MM-VA-1K. For 8.5.0.0 this requires the following (verify this with the documentation).
Total vCPU = 8
Memory=32
Flash/Disk=32
The memory size must be at least the same size as disk 2 (flash/disk) and up to a maximum of twice the size of disk 2 (use the docs to set the memory and disk size). Change the number of CPUs and the amount a memory allocated to the VM, based on the requirements in the docs. Make sure you "Reserve all guest memory" so you do not have oversubscription. As for network adapters, don't make any changes.
The next step is to change the size of the hard disk. This seems tricky, but really isn't a bad process. First, you need to add a new hard disk (hard disk 3 most likely) to your VMM and set the disk size to the new size requirement (probably best to create this in the same storage location). Configure it as "Thick provisioned, lazily zeroed".
Before rebooting, check your disks to make sure that the SCSI IDs are as follows (they should be at these settings automatically):
Hard disk 1 = lowest SCSI device
Hard disk 2 = 2nd lowest SCSI device
Hard disk 3 = Highest SCSI device
After the CPU, memory, and disk changes have been made to the VMM, power on the VMM. The VMM boots from the first and second hard disks by default. If it sees a third hard disk, it will automatically mount the new flash (hard disk 3), format it, and then copy the files from the existing disk (hard disk 2) to the new disk.
After this transfer is complete, the VMM will finish booting from the new disk, and use this new disk from now on. You can now login, and the "show storage" command will show the expanded disk size. The "show inventory" command will now show you are operating as the larger machine.
Before going on your merry way, you do need to do some clean-up just in case for the future. You should first delete hard disk 2 (remember, you are now using disk 3, so this is technically sitting there doing nothing). After you delete hard disk 2, while the VMM is powered off, change the SCSI addresses. Hard disk 0 should be SCSI 0:0 (you should not have to change this) and the new disk (which was hard disk 3, but is now 2 because you just deleted 2) should be set to SCSI 0:1. If you ever had to do this whole procedure again in the future to expand your MM, if you do not do this, you will not be happy when you try to do this process again.
Please be careful doing this. I have done it multiple times, and it is a straight forward process, but I can't guarantee things have not changed (I don't think they have).
At this point, you should have a larger MM-VA, able to support your additional devices.
I hope this helps,