Just like when you walk in, if you port into a square that is next to a monster they will attack automatically. This is already in fheroes2 and it works well. If you want, for already placed portals on the map you can exclude this as a default and you arrive next to a monster and make it an option. But since you can always reload if you port next to a monster (especially with DD and also with regular portals) I don't think it breaks the game except for maps where you are meant to come out of a portal and avoid a monster group on one side (basically the H2 equivalent of one-way portals).
So I would suggest: have auto-attack when you DD next to a monster for all games, and have an option for auto-attack (default: off) for when you are on a portal square and you arrive next to a monster group.
You could still create one-way portals for new maps even with the placed portal auto-attack option on, as you simply leave a space between the portal and the monster so they don't auto-attack, but you still can't exit the portal without a fight. But if would make sense for it to be off for current maps.
Protecting their treasures should be a given for creatures, so for DD they should always auto-attack if you land next to them.