You sound passionate about the BHSC program, I hope you get in!
Here's some info from their admission page:
A combination of the supplementary application ranking and final grades with high rankings on both elements will determine if an applicant will receive an admission offer.
Applicants interested in being considered for Level II must have completed six units of university Level I Biology and six units of Level I Chemistry (equivalent to Biology 1A03 & 1M03 and Chemistry 1A03 & 1AA3).
For grades, it just says a minimum 10.0 (A-) on a 12.0 scale GPA/average
They do accept applicants from other universities:
Level II Transfer Applicants from McMaster University:
Apply through Mosaic. Level II Program Application available in April (refer to the Registrar’s Office website for specific open and close dates).
Level II transfer applicants are not eligible to apply for a BHSc Specialization (ie. sub-plan) in Mosaic. If you apply for a sub-plan your application will not be reviewed for BHSc (Honours) and you will be considered ineligible.
Here is some more helpful info:
Information for All Level II Transfer Applicants:
Transfer applicants are ineligible to apply to a BHSc Specialization. If a Level II transfer applicant receives an admission offer and would like to be considered for a BHSc Specialization they will be considered on a case by case basis, only if space permits. There is no guarantee that space will be available in the specialization. Therefore, it is critical that if you apply to BHSc (Honours) that you are interested in the core stream.
We do not consider applicants to this program if they have completed a University undergraduate degree or if they have completed more than one year of University undergraduate studies.
Supplementary Applications will be reviewed for those applicants who satisfy the grade and course requirements.
Applicants will be selected based on both their academic qualifications and their scores on the BHSc Supplementary Application.