>>> I don't want to add it by logging into the host and using the vmid. How do I mount it by logging into the specific VM?
The new virtual disk needs to be added from the host side, either through the GUI, command line, or API (which you may be able to call from within the guest).
However, mounting a new virtual disk to a VM is not much different from adding a new HDD to a physical system. Unless the new HDD is connected to the system, you won't be able to use it.
André