You cannot tell the app that you have another hero once a campaign begins.
I'm unsure if anything changes once a campaign starts based on the number of heroes, so you may be able to get away with simply adding that player and using the appropriate group limits for monsters.
Morale is another thing to keep track of. You can probably work something out.
I would be very surprised if there weren't little things that change (boss bonus health, moral increase in cities, maybe even some mission rewards, etc).
Obviously monster groups you have full control over, so that will scale easily, but here's what I'd do:
1) The in-app hero icons are now mostly turn counters. When any hero ends his turn, click "end turn" on any in-app hero. When the round is over (all monsters activated, but before peril hits) take the final hero's turn. It's a slight edge for the monsters in uncommon situations, but not bad.
2) If there are specific events that effect the "highlighted heroes" or "select heroes that fail a test". Keep track of that by hand for the out-of-app hero. Usually it's easy to tell what the peril or event is referencing, so just keep those things in mind and play as honest as possible in those situations. If everyone passes a test like this except the out-of-app hero, select an in-app hero as proxy so the later perils are still triggered.
3) Give XP to the out-of-app hero equal to that earned by the lowest XP in-app hero.
4) Judgement in favor of the monsters. If your curious about whether or not some event or monster placement is affected by # of players or not, rule in favor of the baddies. (This monster has +9 health for 3 players, is that actually +3 per hero? Sure.... it's now +12)