When a PCB is reverse engineered from Gerber files, it is full of DRC violations, because KiCad does not know anything about it’s netlist, except the connections implicitly implied by the existing connections. You can set: PCB Editor / Route / Interactive Router Settings to both Mode / Highlight collisions and Allow DRC violations to make extra connections.
It has been quite a while since I wrote:
In that article I deleted the zones, re-created the schematic and replaced the pads with real footprints in further steps, and then let KiCad create new copper zones. Manually editing the copper zone to make room for the extra track will be difficult and/or cumbersome. But there is a lot of variation on how zones in gerber files are created and it depends on that. You probably want to tweak other settings of the interactive router too when you attempt this.
It is possible you can make use of the Wire It plugin. (I have no experience with it myself, and it is not available via the plugin and content manager).