Can I somehow add an editable "From" field to Mail.app, a bit like Virtual Identity for Thunderbird, just like I could change the "Reply To" address on the fly?

In Mail.app, one can set up multiple email addresses for a single account by just entering a comma-seperated list like [email protected], [email protected]. Next, when composing a message, Mail offers a dropdown to select a "From" address. And when replying^†, it automatically selects the right address if it can find a match:

Nice, but I'd like to be able to change the "From" on the fly, without going into the Account Information.

Also, in previous versions of Mail one could even specify multiple Full Names for a single account:

Email Address: Arjan <[email protected]>, Arjan on SU <[email protected]>

But nowadays, Mail only uses the setting of Full Name, and even ignores names in the Email Address field when no value for Full Name is set at all. Hence, it would be great if I could change the Full Name on the fly as well.

I had no luck finding a plugin for Mail yet.

^{† When using sub-addressing, anything that is sent to [email protected] is simply delivered to [email protected]. I'd then like to reply with the same full address, rather than just [email protected] if Mail cannot find a match, without going into the Account Information first. I sometimes also want to compose a new message with a new sub-addressing-address.}