cd to where the .vmx (the vmware server config) is stored for the vm you want to add the disk to. Create the disk. vmware-vdiskmanager -h gives what the options are, and are pretty self-explanatory.

vmware-vdiskmanager -c -s 1Gb -a lsilogic -t 3 .vmdk


From what I can see, your VM must be running to do the next two
commands. (Even though if you do it through the console, you have to shut down your system)

Next, attach the disk image to the VM, in order to do this properly,you'll have to know which ID's are available on your scsi tree:

vmware-cmd /config.vmx setconfig scsi3:1.fileName custom-2.vmdk
Finally, add the disk image to your command

vmware-cmd -v /config.vmx setconfig scsi3:1.present true

Finally, verify that the configs were actually written properly:

vmware-cmd /config.vmx getconfig scsi3:1.fileName
vmware-cmd /config.vmx getconfig scsi3:1.present

After I added them, I rebooted my VM just to be safe.