2. I (as a purchaser) signed the provisional agreement but I want to add my spouse's name or my parents' names into the subsequent formal agreement. Can I do that?

Yes, you can. Before signing the formal agreement, your solicitor will ask you and your spouse (or your parents) to sign an additional document confirming that you were being nominated to sign the previous provisional agreement on behalf of the "new purchaser". The "new purchaser" includes you and your spouse (or your parents). This document must be presented to the vendor's solicitor for preparing the formal agreement.

Additional stamp duty on the sale and purchase agreement is chargeable if the person joined as the purchaser is not your spouse, parent or child ( section 29D(6)(c) of the Stamp Duty Ordinance ). You should check with your solicitor on this matter.