I haven't seen a grid2pos function anywhere on the interwebs, so I thought I'd make one which I was also planning to use in a artillery dialog.
You can use it by putting this in your init.sqf:
mk_grid2pos = compile preprocessFileLineNumbers "mk_grid2pos.sqf";
And then you can call that function like this (accepts a string as argument, returns a 2d pos array):
["0285717619"] call mk_grid2pos;
You can look at the screenshot to see what happens if you call it like that (screenshot was taken when debugging mode was enabled, so normally there wont be a marker of course).
It currently only works for 6, 8 and 10 digit coordinates, and has been tested in ACE and ACEIP. Only islands listed in the function are supported because doing grid2pos and pos2grid conversion are kinda dodgy in arma.
It also does not support islands with alphanumeric characters in the grid, but islands shouldn't have those anyway (even bis is realizing that these days).
Btw, special thanks to Spooner, because without the code he posted about his pos2grid solution I would've dropped this idea as soon as I found out the obvious problem with the Y axis.