Sometime after June of 1916 about 500 infantrymen were transferred into the flamethrower regiment to fill the ranks of the 3rd Battalion.
In late 1916 the light trench mortar was designated a weapon of the infantry, since the pioneer arm couldn't provide enough men to serve as trench-mortar troops. It could be that the pioneer arm couldn't provide enough men to serve as flamethrower troops, either.
Judging by the date Kämpfe went into the 2nd Guard Pioneer Replacement Battalion, he may have been one of those 500 infantrymen recruited as flamethrower pioneers.