BeerBorg

Gentleman, I have one MrB Porter and almost a full bag of Caramel40 grains, if i steep the bag of grains for 20 min, add the MrB can....what will the caramel do to a porter? lighten it? is this a terrible idea?

How much is "almost a full bag"? You need to weight it.

Adding a small amount of Caramel 40 wouldn't hurt it - like 4oz. It would lighten it, but it's so little that it won't light it much.

If you haven't made American Porter by itself, I wouldn't do it.

I'm not a porter fan myself, but I'll pile on to what Rick said: If you haven't brewed the recipe straight-up yet to determine a baseline before making additions, I would strongly recommend you do that.
If you have, please ignore this post.

If you have never tried this porter before, I would most likely not add it. A small amount like 4 oz steeped most likely won't hurt it and maybe give it a fresher flavor.. If you want to play with steeping grains you can't almost go wrong with 4 oz of carapils in a MB size batch....A good steeping grain to give your beer good head.