Well, for a experienced PHP and Joomla developer a few hours maybe, since you need a backend to control the connections and some modifications in the compose message logic.
The biggest problem is that when you modify the code on your own you leave the development branch and all bugfixes and improvements must be added by comparing the complete code basis. uddeIM is the most complex PMS for Joomla, so I do not recommend to add patches without adding these to the main trunk otherwise updates/upgrades won't be possible in future.
For a service you describe are most features of uddeIM not necessary, so maybe it might be more wise to use a very simple component and add some modifications there?
uddeIM & uddePF Development
CB Language Workgroup
CB 3rd Party Developer