You can add a subscriber by forwarding a message.
Note: You must be able to forward a message unmodified (i.e. with the headers unchanged) in order for this to work; otherwise you will end up adding or removing yourself from the list.
First, you set up a program alias :
imailsrv -add domain listname
For example: imailsrv -add exotic.birds.com Parrots
Then, to subscribe a user by forwarding:
Forward a message from the user to the alias (Parrots_add), and the original sender of the message will be subscribed.