Question 8 of Application to Sponsor and Undertaking asks you to, "Enter the date (year, month, day) you were married or you entered into your current common-law relationship." Note that this is actually the date your status officially changed from being single to common-law, not the date you started living together (I think I may have misinformed someone before.) The sponsorship guidelines imply that it is at the time of applying that you need to have been cohabiting in a conjugal relationship and have done so for at least one year. First of all, this is part of the undertaking forms, as I already mentioned. Second, you are supposed to send a full and complete application package at the initial time, not as it becomes available. Since one of the aspects of sponsor eligibility is sponsoring a member of the Family Class, it follows that the sponsorship undertaking may be refused on the basis of not being applicable to a member of the Family Class, which the immigration officer would determine when he or she sees that you are applying under the common-law category and have entered a date in the future for question 8. You could try though but you may get a letter about it from Buffalo anyway. I don't recommend lying to CIC either.