Must I Unlock Treasure Maps Before I Can Get New Treasure Maps Via Canvassing / Tag Mode?
No. You can receive and use treasure maps as soon as you are able to Canvass for Guests at the Quester's Rest in Stornway.
If you receive a treasure map (whether through canvassing or the normal quest in Zere Rocks), you can share it through canvassing as soon as you have beaten that treasure map's boss.