It's not very easy to create a custom handler, since you would need to add it to ALL SIM.CFG of ALL vehicles, and you'll have to edit all texture.cfg fallback paths to be sure every vehicle can find the new texture, which of course must be made correctly according to the SDK specifications AND it the package layout must be updated to include all the files that has been added or modified.
And, of course, you need edit the rules files to assign it to some airports as well. I suggest reading the last pages of the manual, the "Operator assignment rules", which explains how the scoring system works.
It's really complex, and all your work will be wiped out anyway at the first update so, it would be much better if you just said which operator you are missing, with some pointers where they operate, and we might add it quite easily ( because we wrote a software that automates that and integrates with the whole build and the update system ), for everybody's enjoyment.
