Hi @pedro_prada, Michael from Asana here!
Throughout the life of a template there will be lots of change in the content of this Project, which is meant to echo the needs of our business & personal use of them. As such, the core template used, which when copied, is where we make those changes for use in the future. This is the benefit of having a template that is flexible to change.
If you have already copied the Template project and require additional Tasks to be added within it, you must do so directly in the copy itself; not the original Template. Making a change in the original Template is how you ensure that those new Tasks/Subtasks are included in all future copies, not the ones already made.
Hopefully you wont need to do this retroactively too often. A not so savvy workaround for this would be to copy the relevant Tasks and add them to the already copied project for your colleague to complete. Depending on the privacy settings of the copy of your template, you may need to have them add you as a Project member for you to manually add the updated Tasks/Subtasks in question
Hope this helps!