is this possible?
 

is it possible to add a distance meter to any window,(target window specificly would be great) that tells you the distance from your current target?

Well, it's possible to add a meter. Just not a working one, as Sony does not publish distance to target data to the client via any usable UI variable. But if you just want a line of text locked at say "24.8m" that's easy to add. :P

The closest thing to what you want would be to fill a hotbar with items/spells in a progressive range basis... i.e. have a 10m item at one end and work your way up to the longest range you have, and that would give you a rather crude distance measure. (I use the thermal shocker for some distance measurements, for example).