I love the pattern you linked to. My thought immediately was to decrease teh width of each stripe to allow for placement of the additional stripes without changing the overall dimensions of the quilt. Otherwise I would suggest that you increase the dimensions of the quilt, both width and length, by amounts equal to the additional stripes. Personally I think the quilt would look too out of whack if you just added more stripes to the length. Good luck whatever way you go.