I too only heard of King Sagar's wife Keshni (the other being Sumati)...I haven't heard of any other Keshni in Hindu Puranas. My thinking is that JSK probably combined Asti and Prapti for some reason and named the new character Keshni....either that or the character Keshni really does exist in some other version of Krishna Leela.
I remember long back when new Ramayan was airing, we discussed all the different versions of Ramayan in the Ramayan Forum to pinpoint where all the strange new stories came from that were not in either Valmiki Ramayan, Ramcharitmanas, Adhyatma Ramayan, or any other famous Ramayan. I remember RF member Khalrika told us a lot about so many different versions of Ramayan that she owns at home, and that many of the stories new Ramayan showed were not made up by the Sagars, but taken from those Ramayans which are not that popular but do exist in different tribes and regions in India.
So perhaps the character of Keshni does exist in an unpopular but still existent Krishna Leela source somewhere in India. Though Asti and Prapti are described in detail in Bhagwatham and a lot of other popular Shri Krishna sources, and are accepted by scholars as the two wives of Kamsa, we do not know exactly what sources Sagars used for JSK.
I too did not see the earliest episodes of JSK (only started when Dhriti entered the serial), so I do not know whether Sagars showed Keshni as the daughter of Jarasandh, but if they did not...perhaps Keshni was the 3rd wife of Kans or something? I find it hard to believe that someone like Kans would stop at two wives...he probably had a ton more like Ravan and even a great number of concubines, so perhaps Keshni was one of them.