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.
![Wink ;)](https://mibew.org/forums/Smileys/default/wink.gif)
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.