Possible to modify expiration date of role using rules?

Is there a way within rules to modify the UC role expiration date for a user?

Specifically, users will already have paid a membership on an earlier non-drupal site. I can calculate how many days they have left from their old membership using rules, and would like to add those days to their new UC-purchased role after that role is granted.

I'm not sure what variable I would be setting in a rules action, or whether it is even possible to set the role expiration this way.

Any help would be greatly appreciated!

Jonathan Sibley