At our lake, the wild mallard DO come up to you and will eat out of your hands. But, all of those that are the most tame are ones that have lived at that one lake their entire lives, so the see people as a source of food and not a threat. Not too many migrate from this area.
Do you go to this lake often? Where they there the last time you went?
I would talk to whomever is in charge of the lake about them before taking them, just to make sure. Where I live, there are several dumpees and a lot of people would be extremely upset if one was taken. If someone wants to rescue one of them, I usually advise people to take them right after they are first seen at the lake.
For now, I would get some food and go there to feed them as often as you can until you can find out more information.