There currently isn't a way to move multiple add-ons and themes between collections at once. This makes it difficult for people who have saved many pages of favorites to organize them. One way this might work is when you edit the add-ons in a collection, you could select the add-ons you want to move via checkboxes, then select the collection you want to move them to from a drop-down.
Thanks for filing this. Due to resource constraints we are closing bugs which we won't realistically be able to fix. If you have a patch that applies to this bug please reopen. For more info see http://micropipes.com/blog/2014/09/24/the-great-add-on-bug-triage/