There is easy no way to d this from the Recovery Manager for Exchange Console. You can create a new Target Storage or Source Storage and re-select your mailboxes. Or, proceed with the workaround below:
Note: The following instructions are for changing the Source Mailboxes so they are listed under the Target Storage.
1. Open the Recovery Manager for Exchange Console
2. Right-click Target Storages and click Add Storages...
3. Click the type of Target you require. If Online Exchange click "Access online Exchange"
4. Add any mailbox (this is only temporary so we can generate a new Target Storage file)
5. Close the Recovery Manager for Exchange Console
6. Navigate to C:\Program Data\Quest\Recovery Manager for Exchange\ and make a backup of the Lists folder
7. Open each .lists file in Notepad, until you locate the correct mailboxes you wish to move from the Source Storage to the Target Storage.
8. Once located, find the newly created Target Storage .lists file with the mailbox you added.
9. Open Notepad as Administrator, and browse to the newly created .lists
10. Delete the contents within the file, and copy and paste the contents of the Lists file you need from Step 6.
11. Save the .Lists file
12. Open the Recovery Manager for Exchange Console
13. Your Target Storage should now include all mailboxes you added previously as a source. You can delete the Source Storage that you no longer need.