I don't believe you can reassign party1-n in an addon, but you can create virtual buttons that do the same thing based on your multibox team composition. For example:

Code:
/click TargetToon1
instead of

Code:
/target party1
If that sounds appealing you might check out my mod ActionBarTargeting. Feel free to steal and modify the code or to request additional features if what I have now doesn't work for you.