With the upcoming Scenery/gateway hub there will be a way to modify stock airports, but in the meantime ,
You need to create a new project to add stuff or modify an existing Airport
If you want just the cosmetic part (add apron/polygon for the taxiway and a painted line for the taxiline) you can create a empty project with an airport and add the stuff you need, it will be loaded on top of default
The added scenery, as described, won’t have the AI functionality (because it is disconnected from the default). If you need that too, you need to redo the whole taxi network (so called taxigraph/AFCAD)
There are some tool on line if interested
The official discord has a SDK channel
I have one too if interested