According to Anchell's The Darkroom Cookbook, 2nd ed, page 254, the ingredient that does the job of helping diffuse fixer out or paper and into the HCA solution is sodium sulphite. Sodium bisulphite is described as an optional ingredient, used to lower the pH in order to prevent softening of the emulsion of film. Anchell further suggests that if paper is to be used, the bisulphite may be left out in order to improve the paper's gloss.
The recipe from the book is:
Water at 52C, 750 ml
Sodium sulfite, 200 grams
Sodium bisulfite, 50 grams (optional for paper)
Water to make 1 liter
This makes stock solution. To make a working-strength HCA solution, dilute 1 part stock to 9 parts water.