Actually, that's always been there!
First of all, the important thing to note is what the "dead" condition signifies: that the race no longer has any planets (and all their outposts are destroyed when this is true). However, it is possible that they still have some living members of their species elsewhere, and can later retake a planet.
There are a few ways for this to happen. Off the top of my head, but not limited to:
1. On other planets, there may be expatriates of the race. These actually don't tend to repopulate, but it's possible it might happen (particularly if the POWs are freed).
2. There may still be pirate bases belonging to the race in question. They may be attacking a planet along with another race, and when the other race captures the planet they may turn it over to the pirates if it is an honorable race (Skylaxians or Andors) AND the planet being captured originally belonged to the pirate race.
3. There may be resistance fighters or POWs of that race on a planet they lost. In the case of POWs, mostly they will just be left in captivity and gradually die, but you can run missions to set them free as resistance fighters. They will also be set free by allies that capture the planet (and may then be given the world). Resistance fighters, on the other hand, are actively dug in and fighting, basically guerrilla ground troops. The planet may be lost, but they might retake it on their own if they are strong enough. Also if you help them out (there is a dispatch for that), then that can also help them retake a planet.
Note: Thoraxians do not take prisoners, and actively scour the entire planet of everyone when they capture it. So you won't see POWs on those planets. You'll also notice that no races surrender to Thoraxians, because they literally can't. The POWs come about when a race sees that they are doomed and surrenders their planet.
The inevitable followup question: Why the distinction of a race being "dead" when they control no planets but still have some members alive? Well, from a solar stage standpoint, the race is indeed dead. They can't make race-wide decisions anymore, they neither gain nor lose attitude or influence, they no longer "count" for anything to do with forming the federation, and so on. It's actually possible to have a "only one race is alive" victory where there are some minorities of other races still alive. Never all 8 races, because Thoraxians neither surrender nor take prisoners.
