I ad eggs to existing hatches all the time with no problem. I've read a lot against it but that was after I'd done it a bajillion times without having any problems and 90+% hatch rates.
I have eggs hatching in two weeks, and I'll keep you updated!!!! There's 10 Polish, 2 Seramas (My first ever!) four Silkies, and 6 Mille Fleur Cochins! It's a shared hatch, with three hens working together. Two of them have hatched together before, the other is a rescued frazzle (frizzlexfrizzle) who really wanted to hatch so I put her with my two Super Moms to make sure her eggs were covered, since she's all bald and everything...All three hens are Silkies, and this will be the fourth batch for one of them, she is an AWESOME mama!