You can do it in the same way Hooktheory used it in some of the examples in their book; by forking this Theorytab (use the Nocturne in Eb one linked below), and then selecting the notes by left / right arrow keys, you can clearly see how rest notes with negative durations are employed to align notes on the melody track as if they can overlap. In particular, by copying the negative semiquaver rest note in that Theorytab and then pasting whenever necessary, combined with the fact you can reach over 7 octaves with the Jazz plugin, you can literally achieve polyphony with no limitations.
I recommend finishing the lead, and then pasting those negative rests and tying them into one rest note that spans over the entire row, and then doing the counterleads row by row. Proof of concept.
Obviously this feature has not been officially endorsed by Hooktheory themselves for nearly four years, who seem to have manipulated the underlying Theorytab data format directly, so use with extreme caution. For example, it seems to crash while exporting scores, and simply clicking on the melody track to position your cursor will most likely break, so instead click a note and then click the same note again to position the cursor immediately after it.
You also cannot use it with other Theorytab-exclusive features (inverted sus4, augmented fifth, added fourth) unless Hookpad allows pasting data across different opened Theorytabs.