This is exactly what I did. I was in SDS and did their 1 year BSW. I had applied to it as well as UofT 2 year MSW. I was rejected from UofT but did the BSW at Waterloo. I think it really helped as now I have been accepted to both Toronto and Laurier advanced standing (funny enough, I was waitlisted at Waterloo).
If you're currently in your BSW you can apply to advanced standing programs, you'll just need to submit your transcripts and maintain a certain average to hold your spot if you are accepted into a program.