Can I fix under-cooked rice by adding liquid to the pan when I make my stir-fry?

I know you can add under cooked rice to soup and then let the soup simmer until the rice is adequately cooked. I have doubts about whether high temperature cooking would produce good results.

You might do better to cook it more in a pot with water before you stir fry it.

No!  You need to add water, but don't do it when you're making the stir fry.  It will steam the veggies and meat, and do weird crap with the egg (if you use egg).  Instead, just put the rice in a saucepan with some extra water and cook til water is gone.  Add more water if it's still not done.  You could also do this in the microwave, but the one time I tried that, it came out kind of weird. 

Boil it in water for another two or three minutes, then drain it in a wire mesh strainer. Just one of the recovery skills you will learn as you cook.

Yes, add liquid to fully cook the rice 

You can try steaming it for a bit. I have tried to save large amounts of undercooked rice by adding water and reheating-sometimes good, sometimes bad-depends on How undercooked it is.

Try steaming it.  Boil a cup of water at the bottom of a pan, put the rice in a sieve, place in pan, making sure that water and rice are not in direct contact.  Cover the pan lid with tea towel and cover rice, leave for about 15 minutes simmering.  Will be great with a stir fry.

Extra Tip: Try putting a sprig of coriander on top for some extra flavour.

You'd have to do it RIGHT NOW and refrigerate it until it was cool (not necessarily cold).  "Fresh" rice makes awful fried rice.

Boil it again for a few minutes.