Yes it's okay to add some oil to your engine between oil changes if your vehicles engine gets low on oil.
Just be sure to change the oil at 3,000 to 6,000 miles along with the oil filter to help keep your engine operating properly and extend the life of the vehicles engine.
But you certainly can add some new engine oil to your old engine oil in the vehicles engine without a problem.
I've done it for years and never once had a problem with the engine when doing so.
I just make sure to change the oil and oil filter after 3,000 to 6,000 miles which helps to protect the engine.
I also change my my transmission fluid and transmission filter every 3 to 4 years or 25,000 to 40,000 miles when I have time to do so.
Changing the transmission fluid and transmission filter helps to extend the life of the transmission as well.