Oh... I think I see what you're saying. You want to rescue new birds to give them a long happy life rather than let those birds go to someone else who will likely cull them at a young age. Yes, that would certainly be preferable,
if the birds you bring in wouldn't be at risk. Hence the conundrum.
If your older birds have recovered from an earlier IB outbreak, they are now likely immune, but still carriers... so any new birds will be exposed to the disease. It's extremely contagious, but once an area is cleared of birds, the virus cannot survive long, a week maybe (very unlike Marek's that can hang around for months or years). That's why it's advised to cull the entire flock and start fresh, because IB is unlikely to return, as long as there is excellent ventilation in the coop. Ventilation is often a very important overlooked factor. You may want to switch to an open air style coop or replace a wall with hardware cloth.
So since new birds would be exposed, they have a good chance of contracting the disease and suffering respiratory and reproductive tract lesions and possible death (which seems to have happened before) even if it does take some time to occur. Though it's possible they could recover with supportive treatment and just become carriers, but there is no cure. If I were you, I don't think I would bring in new birds, as their rescue would probably just lead to suffering.
You are in a very tough position and I feel for you. No one should have to make such a difficult decision. You have to decide for yourself to either let your current birds live out their days as a closed flock, or end their suffering now and start over with healthy birds that have a good chance of outliving the birds you have now. Heartbreaking either way, and I'm so sorry.