At the moment there are no such feature in Mibew Messenger.
I see two possible ways to do so. Both of them aren't simple.
First one is to rewrite some parts of the core of Mibew. See
libs/classes/Mibew/Mail/. BTW, if you implement this feature and send pull request, it will be possible to include it in the code base.
Second one is not related to Mibew Messenger itself. One could implement some kind of filtering tool on the server side to change
"From:" mail header depending on a value of
"To:" mail header before actually sending an email.