Ok, here is my comment
But TtTOtW is right, I'm not a specialist into planet editing. But for what you want to do, I guess you just have to edit the parent in the ORBIT_DATA section.
For example:
●ORBIT_DATA●
{
"parent": "Jupiter",
"orbitHeight": 75200000.0,
"multiplierSOI": 1.25,
"orbitLineResolution": 100
}
This is the ORBIT_DATA section of Callisto. What tells you it orbits Jupiter is the "parent" parameter. You can set any existing body here (so you can make a moon of a moon of a moon...).