In the late 40's to the mid 1980's the PRC seem to have never thrown any firearms away even if they were unusable by western standards. I remember one firearms references calling it a logistics officers nightmare to supply all of the weapons from enemies, allies, captured, stolen, surrender, locally produced or otherwise used by the PRC. I remember in the 80's all of stuff being sold that was imported from the PRC, mausers, broom handles, m1903s, mosin nagants, Browning Hi-Powers, revolvers, etc. Some were in good condition most were just junk that required major rebuilds or just scrap. I remember parts sets for the Chinese built Maxim guns, Thompson converted to 7.62mm Tokarov, M1919A4 converted to 7.62x54mm Russianicon, Colt M1911s to 7.62mm Tokarov. and many others including the Japanese Arisakaicon rifles converted to 7.62x39mm Russian.
I imagine an insert can be made and fireform into the chamber of the Type 99 Rifle. Of course the ejector and extractor would have to be modified for the small base diameter of the 7.62x39mm. A spacer in the magazine case and a modified follower and small leaf follower spring would be appropriate. For that matter you probably could modify an AK mag to fit if you want to be creative.