It's not possible, sorry. Expiration timers are fixed and cannot be removed or extended.
You may want to have an alternative, such as triggering the expiration and giving the unit a buff to signify that it is going to die, or whatever.
If it is a simple unit (a unit without inventory, or buffs, or whatever), such as a dummy for a spell, you may want to remove the unit and then recreate it (and attach the effects or w/e).