Is it a good idea to add oxidizer to my septic tank?
If your tank and box are completely filled, your drain field is plugged up and no longer draining properly. It is not in jeopardy, it is done. While it could have been worse, it is still bad. Unless this was caused by a temporary overloading condition, or the main line was simply blocked at one point, you probably will need to build a replacement field soon, if not now. It's conceivable, that giving the field a good long rest with little or no waste load, that some sort of product that accelerates waste breakdown could help temporarily get the field partly working.

But if the field isn't draining, the product will not get to where it's needed. Some of these products are snake oil and do nothing. Others will help accelerate breakdown, but given enough time, breakdown will occur naturally, special products are not needed, but may be helpful. Every guy with a pumper truck sells something like this because it's easy money.

If your field is partly draining, and this Oxidizer Plus (many similar products around, btw, shop around) stuff really is actually beneficial, it may be worth a try, as the real solution is very expensive. It will be important to drastically reduce waste loading once a product is distributed to let it breakdown the obstructions with out adding on more work.

The same results can be achieved with no product at all, but it may take longer. If you do get the field draining, it will still be in a very fragile state. It could be easily overloaded again with even modest inflows. You should also determine why the field became obstructed in the first place. A properly sized, constructed, and maintained system should last indefinitely.