your best bet would be to do it from the start... obtain a Barred EE from one of the Barred Olive egger projects(many are doing this on this forum) get a male(rooster)...
Mate a Barred Olive egger to a Welsummer hen... ALL of the chicks will come out barred but you wont be able to sex them at hatch. but thats ok because you want Welbars and this F1s wont be as close to welsummers and future generations.. out of the F1 obtain the best looking barred females that you know is laying green eggs, breed them back to a welsummer rooster. hatch at least 50 eggs so you have plenty to chose from
now on this BC1(back cross to welsummer) be on the look out for wildtype chicks(chipmunk looking, not black) cull the black chicks without headspot and chicks without pea combs.(all females and single comb birds) keep the chipmunk and black ones with a headspot and pea combs.. Only use the black chicks with headspot and pea comb IF and ONLY IF. all of the chipmunk(wildtype e+) males are not Crele looking, IF you are able to get a few males that turn out to be Crele. then cull the black barred males..
at this point your Crele pea comb males have one copy of the Barring gene and one copy of the blue egg gene(when mixed with brown egg shell will produce green eggs).. this means when crossed to welsumer hens(to get to welsumer type and egg color) you will only be able to hatch 50% crele females and 50% crele males, so you need to hatch about 30 eggs. at this point you can chose to breed Crele female and crele male to obtain double barred males, they will be different from the single barred males as they will be Much lighter. so be on the look out for them. keep a few of them. cull the none barred males and females...
after obtaining the double barred males(light crele) you will have yourself true breeding Welbars. Good luck
