In the expansion you can upgrade your spear. You can meet Kamut in the camp near the hot springs located close to the road to Song's Edge. He will give you A Secret Shared quest. During the quest you must enter the abandoned drone station on the north of the map. There you will find an item that you must bring back to Kamut. It will unlock slots for modifying the spear.
Spear modifications work similarly to other modifications. Simply open the crafting tab and then select the spear from the available weapons.
The only difference is that they need special type of modifications. It is represented with a triangle.