I have done it in the GECK, and all you have to do is change the weapon's ID. If you double click on a weapon in the editor it will bring up a page on which you can change the stats of that weapon. In the upper left hand corner of that page it will say 'ID'. Change that to whatever you want it to be and when you click 'OK' It will ask you if you want to create a new Form, select 'YES' and the new moded weapon is now available under the ID you gave it.
Precaution! If you do not change the weapons ID all the weapons in Fallout 3, under that ID, will be changed to the stats that you provided.
Hope this helps.