Actually, I don't think you can turn off the "easy chords" feature on the PSR-E models or change it to another type of chordal recognition.
I'm not sure I understand what you're trying to accomplish.
If you want to play the accompaniment rhythm without having to play the other parts of the accompaniment (bass, chords, pads, and phrases), you can select the "Stop Accompaniment" option. I believe that causes the split section of the keyboard to play chords using a "strings" sound, but I think you can change it to another sound by turning on the Split Voice and changing it to something else. And if you don't want to use the Split Voice at all then you can set the Split Point down toward the far left end of the keyboard so that the Main Voice and Dual Voice span almost the full width of the keyboard.
Or, if you do want to use the other parts of the accompaniment but you want to play your own "full chords" fingering, just play the chords using as many fingers/notes as you wish. The keyboard will still use its chord-recognition logic to try to determine what chord you're playing so it can change the accompaniment accordingly, but you won't be limited to the just the four "easy chord" chords (major, minor, seventh, and minor seventh). The keyboard should be able to recognize something like 35 different chords, although some of them aren't necessarily "chords" per se (e.g., there are chords for "octaves" and "fifths").