dude, its simple, open up H2X and then go to the weapons tag, click on the weapon you want to change, then go over and click on meta editor, its a plain white box, and when you highlight it, it will say meta editor, click on that and then in the BIG white box, click on projectile, scroll to the bottom, it should say x-spawn, Y-spawn, and MAYBE z-spawn, i havent used it in a while, and in that program you can use negative, decimals, what eva, then click save changes, and at the top - edit/sign map,
unless it is a new map, then sign it with a dif program
the easiest way to do it with dothalo, is to open up the weapon, and PRAY TO GOD that there is already a negative variable in the x,y,z? spawn, then just highlight anything to the right of the negative sign and put in the number you want, i think the only weapon that has a negative though is the rocket launcher, maybe im wrong, and here is the link to get H2X if you dont have it
http://halomaps.com/h2x.html , if that link dont work, just find it in the utilities section of the files forum