Mass Effect 2 Discussion
Well first of all the Geth Antimatter Sniper Rifle and the Incisor Rifle are two different things.
You should first have the DLCs to actually edit them, although it is not permitted. If you choose to edit the stats for those DLCs Mass Effect will not register them as valid DLCs.
You can edit these in the Mass Effect 2\BioGame\DLC directory by opening the folder in which the DLC you want to edit is in, but as I said you shouldn't because the DLC will be useless to you then.
If however you do not own the DLCs and you wish to somehow add them, the only way to do so is to buy them and download the from the Cerberus Network.
If you meant how to get the special weapons(Geth Anti Matter rifle, X-300 shotgun etc.) that can be found on the Collector ship, then check out this thread. From what I've read it has worked for people.
* Updated game trainers and cheats daily
* Get notified when new cheats are added
* Request which games get new trainers
* Priority support with any problem