will they give letter of support of their accepted ones? when will it would be?
It depends entirely on the incubator and their process. There will be several steps involved before a letter of support is issued by an accelerator. For example:
1) Due diligence of your business proposal or validation of your existing business and its potential in USA/Canada, intellectual property ownership, language skills, finances needed to support yourself for a year, reference check etc.
2) Completion of any agreement or contract which needs to be signed with the accelerator - like how long you're going to occupy their premises, and/or any other mutually agreed deliverable.
It is best that you take this up with the accelerator directly. You won't get specifics on this forums, just a general guideline.