You can walk up to any AI aircraft as long as it is user flyable, i.e. has a panel, and "get in"
For the purposes of the screenshots above, I started the sim and picked the car as my "aircraft". The Hunter was parked in an AI parking spot, in this case placed via AirTrafficFX. I then drove the car near the Hunter and used "ctrl-shft-E" to switch to avatar mode. I then walked up to the Hunter, and when close, pressed "ctrl-shift-E" again and was placed in the Hunter cockpit. The car remained where I left it.
In another test I started in one aircraft, taxied to a parking spot, switched to avatar mode, then walked across the airfied to another aircraft. I then got in that and taxied the second aircraft and parked it next to the first, then got out again and got back in the first aircraft & flew off.