will we be able to go back in forth like in the fallout 3 add ons

We couldn't go back in O:A or most of MZ.
And it depends on the DLC and what makes sense for the storyline.
Anyway, we have no idea if we'll be able to or not.

we better be, its a friggin national park

I really hope so, for those of us who don't read wiki entries about every single area in the DLC as we play, it can be heartbreaking to find out you left behind some rad to the max item and can't go back and get it on that character. My stealthy sniper guy is missing the Assassin Armor and I'm chapped about it.