Is it possible to create the G13b9 chord in Hookpad?

I haven’t found a way to do it and I’m a total novice at music theory so not sure if there’s a substitute.

No. Hookpad can only enter chords that can be chromatically transposed to be played entirely on the white keys of a piano, and doesn’t support extended chords beyond add9. G7 is probably your closest approximation.

Thank you for the explanation and suggestion.