Open the directory that contains your Dredmor save files. In Windows it's "My Documents\Gaslamp Games\Dungeons of Dredmor\Save" or something. In there you will find a file named "last". It's just a text file that contains the skill ID's of the last 7 skills you picked for your character. Open it in Notepad. Change one of the numbers to 12 and save the file. Now go make a new game and click the Last button when choosing your skills. Deadshot will be one of them. You can remove the other skills if you'd like and add other ones of your choice. All you need to do is hit level 3, because the skill line only contains two skills beyond the first one.