I'll answer this like it goes for every
CSM recruitment question. Not all member are original legionnaires.
They can have in their ranks post heresy renegades, guys (or girls(!)) from Daemon Worlds that can be as big and tough as a
CSM but not really a marine (e.g. Savona in the Fabius Bile novels.). New recruits made from things like the Daemoncubala. Deals made with Bile and the Consortium. Clones, general warp fethery resulting in multiples of the same legionnaire etc. etc.
All of this applies to
DG just like every other legion. Not all of their gene-seed is useless as the Plague Surgeons tend to what precious few are still viable. In addition in a book I've totally not read (
) the implementation process for the
DG is described a little, as many of the new "recruits" are made from infants implanted with gene-seed and are presumably infected with Nurgle's gifts along the way, unlike many other
SMs wherein the gene-seed is implanted at around puberty.