Yes, you can move the entire Domain and PO Directory structures to another Volume.
Just make sure to update the following...
1. In the Details for the Domain and Post Ofice Objects, the "UNC Path" under the Information Tab.
2. In the Details for the MTA and POA, the "Log File Path under the Logging Options Tab.
3. In the <Domain>.MTA and <PO>.POA config file located in SYS:\SYSTEM (You can determine the exact name of the CONFIG files by looking at your GRPWISE.NCF file. It will be defined there.)
Good Luck!