@{Add clears a multivalue attribute

I have some existing distribution lists that have Reject attributes set on them through the Exchange mgmt gui.  I want to be able to go through and append a global 'termed user' group to all DLs.  However, when I use the syntaxhere, it does not append and only clears the attribute.  What am I missing?

Example:

Get-DistributionGroup -resultsize unlimited |Set-DistributionGroup -RejectMessagesFromSendersOrMembers @{Add="company.com/Groups/Termed Employees"}