I moved the F/A-18 over without issues. I copied/paste the SimObjects\FA-18 folder into P3D then went into the \gauges, \effects, \sound and \fonts and copied/paste anything with fa-18 or f-18 in its name.
I removed FSX from my system over a year ago and I'm speaking from what I remember. Be aware there maybe other folders with required files in them. A critical one is FA18.dll and I believe its in the \gauges folder.
It can be done but you'll have to look for those additional files. in fact I have modded my hornet using the Sludge and the FSXBA mods. I've had no problems with either.