An enemy part can be converted with Dr. Montague's help. But since Dr. Montague is in Dr. Osto's Research or Unsealed Door quest, you would have to clear certain quests to gain access to the right quests.

To convert an enemy part into an enemy weapon, take said part to Dr. Montague in the Dr. Osto's Research quest or Unsealed Door quest. Dr. Montague can be found in front of the door to the Ragol teleporters on Pioneer 2 closer to the teleporter to the Principal.

You can convert enemy parts into weapons on any difficulty. It will not affect the outcome of your weapon

For more information on where to find certain enemy parts, look at this guide.

If you are on the Xbox
You need to clear Dr. Ostor's Research and convert the weapons in the Unsealed Door quest for it to work.