An "append to pasteboard" option per numble's suggestion would probably be the way to go - wouldn't be difficult at all to implement, and that would let you accumulate a bunch of characters without switching into Memo.
An old thread at http://www.plecoforums.com/threads/accumulate-dictionary-finds-into-a-message.2672/
said an "append to pasteboard" function would be feasible. This would still make composing text messages a lot easier than constantly going back and forth between two apps, copying and pasting.