Ah, I've got another idea. Is it possible to recruit enemies mid battle? In Fire Emblem certain enemies could be recruited to your party if you had the right person talk to them and in super robot wars some enemies can also be recruited, though usually in order for it to work you have to do more than have someone talk to them. I guess it would require being able to put in a tag for an enemy unit that will cause a "convince/recruit" command to appear when the right actor is standing next to them. Though I don't know what else.
I could be wrong, but for this you could probably have an event. Make it so that the unit doesn't move unless an enemy is in range, and have an if statement with the condition being a certain character. After that make the enemy unit vanish and have a "character joins" type event where you add a new character to the party. I hope this makes sense. Just wish I could figure out why my turns loop...