Unfortunately, I can't help you on the ink tube adjustment; I converted my VG-640 from 7 to 8 color, but apparently that doesn't require any adjustment to be done. I suspect this is documented only in a full Roland Service manual.
I will give you fair warning, however, that the conversion process is a bit of work, and I'm saying that as a person who doesn't back away from a job requiring some elbow grease. It was a couple of days and a few hundred bucks in solvents and materials to complete a single color change - and *you* might have to do up to 4! You'll basically need to break apart most of the line connections to flush them through with solvent. I disassembled the pumps and dampers/heads to clean them as they both hold A LOT of ink. Even with being pretty meticulous in cleaning, my white was printing with a pink hue for about 1/2 of the bag.
I'm not going to tell you it can't be done (like Roland probably would), but just be aware it's not exactly quick and easy. Just make sure it would really be worth it to you to have 2xCMYK over 7 color. These machines do make some impressive looking photographic prints in 7 color mode due to the fine control afforded by the light inks.