The quick answer is 'Probably not'.  But the rest of the story gets a little complicated.  The U.S. military was never officially on the Korean Peninsula in force, but we were in the neighborhood.  In the first few years of the 20th Century we were in China for the Boxer Rebellion and in the Philippines - our new U.S. Possession for the battle with Moro tribesmen during the Philippine Insurrection.  We used a few Krags in China, but we used a lot more Krags in the Philippine Insurrection and even armed the Philippine native defense force with special Krags designed just for them.
Fast forward and we find the Japanese in both Korea and in the Phillippines and even China during the first 40 years of the Century.  There is a book called "Shots Fired in Anger - A Rifleman's thoughts on Japanese Activties on Guadalcanal."  He opines that the Japanese never wasted any rifles they captured and could reuse.  He claims that lots of British Enfields and even Krags were found in the possession of Japanese troops on Guadalcanal.  If correct, that would seem to indicate that they could have been in Korea as well.
Now the serial number on your Grandfather's Krag seems a bit low to have been among the Krags we had in the Pacific, but I suppose it is possible.  More likely, I think, is that he bought the Krag in a sports shop or department store as surplus before or after his time in Korea.  Was he a hunter?  Do you have any photos?  It would be fun to see the piece and let us all give you our 2 cents worth.  Oh, and welcome to the forum!!