YIIP is a partnership program, and there are in-kind or cash contributions to the project from the Host Organization or Implementing Organization. Interns may be required to share some of the costs of the internships through fundraising, domestic travel, language classes, or an in-kind contribution. The stipend is taxable in Canada. If you receive funds from the Host Organization to top up your stipend, you may be expected to pay taxes at the prevailing rate in that country. All personal income derived from the internship should be declared, unless the source of income has tax-free status.
Some countries have taxation treaties with Canada.
You might also wish to discuss it with the Implementing Organization or an accountant.