Yes, you can share Writable assignments in Canvas Commons, with a few limitations. Currently, teachers who find and copy assignments from Canvas Commons to their own courses will not be able to edit the assignments.
In this way, Canvas Commons is currently recommended only for assessments or other assignments that receiving teachers do not need to customize. For example, if a curriculum director wanted to share a series of distance learning assignments or common writing benchmark assessments and have teachers copy them to their Canvas courses without changes, Canvas Commons (combined with the 'My District' feature in Writable) would be an excellent way for an admin team to share assignments quickly with teachers in Canvas.
We are working on enhancements to our integration with Canvas that would allow the assignment creator AND the assignment recipient in Canvas to make edits to their Writable assignments. Please contact us at firstname.lastname@example.org if you have any suggestions or questions about using Writable with Canvas.