some weeks ago I was looking for C/2016 R2 (PANSTARRS) in KStars but I couldn't find it. I checked the comets.dat and it seems there are only ~443 comets in the list which are all periodical. So I looked up the JPL query and there is a filter on ksutils.cpp:1253
obj_numbered is set to "num" which means there are only official named comets selected:
I just changed obj_numbered is to "all", recompiled and got about ~3500 comets. There should be an option (checkbox?) to add non-periodic comets to the list.