OH! Yes I am so sorry; You can mix with existing items but it requires the item path. I plan to in the future make a document with those so players can use the existing items. For now what items do you want your modded items to combine into? I can grab those for you.
EDIT: I think next update Ill add a command like /path at the end of the Force_Spawn commands that give the Item Paths for modders to pull the official pathing. Better than me upholding Documentation for sure. For now what item you need the path of?