Is there some sort of additive we can put in radiator to seal the freeze plug?

Coolant leaks are typically not to bad to repair and can consist of can be the water pump, radiator or hose but to be sure which is leaking these guides can help us fix it.

https://www.2carpros.com/articles/car-is-leaking-coolant

and

https://www.2carpros.com/articles/radiator-pressure-test

and

https://www.2carpros.com/articles/coolant-leak-symptoms

Let us know what happens and please upload pictures or videos of the problem.

If the leaking plug is located at the rear of the block, and coolant is leaking out the bottom of the bell housing, this is the case, there are a number of these core plugs on the engine, they are part of the block casting process, this one is located behind the drive plate or flywheel, and there is no option but to remove the transmission if it is leaking, make sure that it is replaced with a brass plug as the original plug will be steel, that is why it has finally failed, the brass will last a life time and the extra cost of the brass plug is very minimal.

Mark