I think I'm worried about your tank. A septic tank only works because of the natural bacteria that grows in there. Those chemicals could be extremely toxic to the bacteria in the tank, and you could wind up with a 1,000 gallon useless cesspool. If I were you, I would do whatever is takes to add a Tee to your septic outlet pipe for pouring chemicals into the field. It is relatively easier to just add a Tee outside the tank, and just splice it into your exit pipe. Then later in the year, add your tank risers. You just need a shovel and the 4 inch "T" with a slip coupling & some 4 inch pipe at your hardware store.
Just my humble opinion, I have learned a great deal of respect to septic tanks,