Is this the website https://www.digikey.com/schemeit/project/ ? If so it would have helped a lot if you had posted that.
The KiCad netlist is used to transfer data from schematic editor (eeschema) to pcb layout (pcbnew), there isn't a way to import it into eeschema. Anyway, it does not contain enough data to create a usable schematic.
Practically speaking, the minimum you need to export to allow a user to continue editing the schematic in KiCad is: a kicad project file, a kicad schematic file, and one or more symbol libraries containing the symbols used in KiCad format. That is fairly straightforward, I have code which generate KiCad projects.
If the idea is to do schematic design in an external tool, and then do layout in KiCad, that is somewhat different. The netlist must contain references to KiCad footprints, so you would need to allow the user to make those assignments in the external tool. The footprints must be compatible with the schematic symbols regarding pin numbering. In that case you could export a netlist and necessary footprint libraries, and that could be imported into pcbnew.
It is all certainly doable, the file formats are not well documented but fairly transparent, it is mostly a simple matter of programming.