adding mailboxes to an InPlaceHold eDiscovery

Using PowerShell, is there a way to add, or append, source mailboxes to an existing mailbox search? I unfortunately found out the hard way that doing

Set-MailboxSearch -identity "December2013" -SourceMailboxes user1@domain.com

will in fact REMOVE all the source mailboxes that were already there, and REPLACE them all with this 1 source mailbox--not a very good idea.  So is there some kind of undocumented "-appendsourcemailbox" or "-sourcemailbox ++ user1" sort of parameter that can be used?

I know that if I use the EAC, I can add more, but the in palce holds that I do tend to have dozens of users at a time and I'd like to keep them grouped by holds named "MonthYear" like in the PS code above.