Ah, I see what you are saying. Yeah when you make a breeding project you can record every detail all the from the initial pairing.
In order to be able to start a breeding project you could add the sire or dame from the other breeder to your account and just make a note of who the owner is in the animals profile and even mark the animals status as "on loan" or something.
That way you'll be able to start a breeding project record. Since the animals owner isn't on iHerp they shouldn't mind.