The quick answer is to not actually use the tilemap for collision. Place invisible sprites over your tilemap and set as solid to create collision boxes. I believe this is a pretty common practice.
Edit: It looks like you can also edit the collision map for each individual tile in the tilemap editor: https://www.scirra.com/manual/172/tilemap
[quote:1qnfcefe] A custom collision polygon can be set for individual tiles by clicking the Edit tile collision polygon button in the Tilemap bar, or just double-clicking the tile in the Tilemap bar.