I am in agreement that In D365FO extension of canSubmitToWorkflow still not available if not override on actual table. But there is an alternate way on which I have recently worked and its working perfectly on client side.

Please have look this blog and mark this answer as suggested if this will work for you.

community.dynamics.com/.../avoid-over-layering-in-development-of-workflows-on-existing-form

d365technext.blogspot.com/.../avoid-over-layering-in-development-of.html