Is it possible to associate a secondary email address with an existing *public* key

For example, if I have a friend's public key that's associated with, say, email address john@domain.com, but I want to send him a message at john@other.net, is there a sensible way to do this? Is there a way to permanently associate john@other.net with the original public key?

I realize I could manually encrypt from the command-line (via the original email handle) and subsequently email that content, but I'm looking for something 'usable' that would properly integrate with my email client and other standard GPG/PGP tools.

I also realize it's possible to add a secondary uid/email address to a key you own, but neither is that what I'm looking for.