If you want to move the position of a fret diagram, for example, to avoid collision, or to place it between two notes, you have various possibilities:
1) modify #'padding or #'extra-offset values (as shown in the first snippet)
2) you can add an invisible voice and attach the fret diagrams to the invisible notes in that voice (as shown in the second example).
If you need to move the fret according with a rythmic position inside the bar (in the example, the third beat of the measure) the second example is better, because the fret is aligned with the third beat itself.