I will try your suggestiong, thanks! However, I am wondering if I can override the module's main class instead of directly adding it to the module itself, since it was bought from a third party. I am not sure how to proceed to get the hook installed while using the override principle.
Basically, I have a module for additionnal registration fields. Inside this module is the main php file and a template. Everything works great when I set my smarty variable directly in the main class, but it doesn't work anymore when I override it and try to get my custom changes outside of the module. That's my big issue at the moment...and I would be really grateful if someone could give me a solution for that!