I also use Presta valves, and wanted to add a Slime type mixture, and found an article on youtube I think that explained that by removing the tightening nut on top, and then dropping the rest of the valve into the intertube, where you now need to keep track of it with your fingers, you can then add your Slime type material and then just manage to bring the valve back to the inside of the tube and reinsert it into the valve hole.
I tried this, and then lost track of the core when entering the slime, but was fairly easy to find it again and reinsert it the valve. I then reinstalled the external nut, and now I am all set. I do have a friend who was poking a small hold in the tube, and then inserting the Slime this way, and then patching the hole by normal methods, but I think that I have convinced him that my method is better.