you can change fifth gear yes, since on 085s the fifth gear is just under the black cover on the end of the box you can change it relatively easily without doing a full strip-down of the box.
You'll first need to find out what code your box is and then either find the gear ratios of it on the internet somewhere or work it out by looking on ETKA or Vagcat.com. You'll also need to know the final drive ratio.
You can't really just assume that putting a fifth from an ATV will give you a longer fifth gear, as the full gear ratio is a factor of the gear itself plus the final drive ratio. E.G. G40s and GTs have pretty much the same gear ratios but it's the final drive that makes a g40 box a fair bit "longer". Fifth from an 8P box might work as these are pretty long gears, I know a fifth from an 8P into a G40 ATV box will give it a slightly longer fifth.
But you need to know what actual ratios are in the box you have before you can choose which gear to use.
