I'm sure it is a hassle to permanently create the line set you want, instead of having an adjustable system. It might work by folding the lines to double, but it might weaken the line connections, because the lark's head you make halve way the line is done from a thin line, instead of a sleeve.
This creates sharp turns in the line, which lowers the breaking point of the line.
Since the lark's head is done over a pigtail, it might not be such a big problem, but still something to consider.
As a side note, using thicker extensions I regularly use an adjustable loop knot to adjust the fifth line of my ozone 2006 bar to fit my old north kites (whenever I had to lend my north bar out for instance).
This one does the job, it really holds!
It might suit you as well, as long as you do not try to tie it in thin kite lines.
Would you REALLY want to have a secure knot on kitelines that does NOT weaken the line, IMHO this is the way to go:
http://www.kitehigh.nl/i/gallery/upload ... 0small.jpg
It should just be tied really really tight, that's all. And use plenty of folds (at least 20 or so).
I do not encourage this course of action, but if you must, please use this knot.