Under certain circumstances you can complete one mission after the crew is taken - hopefully, it will be Legion's loyalty mission. The trick here is to make sure you've talked to Legion enough so that you get his loyalty mission and it's in your logs before you touch the galaxy map. If his mission is in there before the crew is abducted, you are able to do that mission, get him loyal, and then save everyone and not just half. Him and Tali, though, I'm not so sure.
See, my MO is to get everything possible done before going to get the IFF, so that all I have to do is say hi to Legion, and then again when he asks me for help. I do this before using the galaxy map after EDI says the IFF has been installed. Sure, it means Legion isn't around for much, which sucks, but at least I save the crew (and if I want to bring him on DLC jaunts, I can do that after the suicide mission).
There shouldn't be too many (if any) alterations to be made vis a vis the DLC and the suicide mission. Sure, if someone's dead then you'll miss out on whatever lines they or someone referring to them might have had, but if, say, Garrus is dead you'll still be able to complete all the DLC. You will be super-sad, however, because Garrus will be dead and that's terrible beyond words. Don't kill Garrus.
As for doing the DLC before the suicide mission, as mentioned above if you do those after the IFF you're asking for your crew to get vaped. All of it can wait until after the mission, or you can do it first thing in another run-through. There are very few functional differences in the endgame with or without DLC.
SO: If you've already lost the crew but you still have Tali and Legion's loyalty missions to do, you either sacrifice half of the crew for loyal members, or you take a chance and hope everyone else's loyalty will keep them alive (don't pick those two for anything special, for example). Installing the DLC will not make a difference in the number of missions allowed before the crew dies, it just gives you more overall to do that does not significantly impact anything outside of the overall storyline and your enjoyment of the game.
(I realize that was a lot of text and probably a bit repetitious. You might be able to fiddle with Gibbed, but I'm not sure what specifically you'd have to change. Someone else may have figured that out.)
(What you can do in future games is fiddle with Gibbed so that teammates like Tali and Legion are available right away. You can't go and talk with them on the ship, I don't think, but you can use them in missions as they'll show up as available in the team selection screen. It's how people are able to make gameplay videos showing what Samara et al say in certain instances, like when you're going to get Jack and that prisoner is getting beaten.)
Basically, ah, you're probably going to want to go back to a different save. :(
Here is a link to a question that may be relevant to your concerns as well. Here's another relevant question and answer that also has an explanation of what you can do and still save your crew.