I’m in Phoenix AZ summer temps 110-120, winter lows 30’s. I have a 40 year old pergola using two 4″x10″ beams suffering from dry rot. In addition to applying wood hardener to the exterior surface is it advantageous to drill small holes (1/4″) down from top about half way and inject hardener.
I’d do a thorough inspection of the wood and remove any rotting wood. But, the answer to your question is yes, you may drill holes and inject the wood hardener, which will enter the softwood and harden it.
It is not a miracle worker, though, and it will not heal rotting wood. My best results have come from removing rotten timber and replacing it with healthy wood.