They will fatten ok, but the carcass size will be considerably smaller, and they will take longer to finish, also will want more feed to get them up to a decent size as they are not bred specifically for meat.
I do a bit of work for a petting zoo and they hatch a dozen chicks every weekend through the summer for the kids to look at, they give me them when they are around a week old, these I fatten for my own consumption, as a rule traditional breed chickens take roughly 24 to 26 weeks to get up to the 5lb mark, compare this to a Cobb-Vantress Cobb500 which can get up to 5lb in 35 days beca