I learned that I can use my transmitter on VRC simulator but it requires an adapter for it. Can I directly do it somehow using a standard mono or stereo 3.5mm audio jack cable (male to male) maybe with some modifications?

You need this adaptor along with a male to male 3.5mm cable to connect it to your pc.

https://www.banggood.in/Flight-Simulator-Wiring-Cable-Plug-For-Flysky-I6-Esky-Transmitter-p-999562.html?rmmds=myorder&cur_warehouse=CN

You can do your own adapter using arduino

I think you can do it yourself with only an audio jack but it will require opening up the transmitter so it's only for those who really know their stuff.
Here's the link if you're curious.

I have a pistol grip transmitter.