How to using API or LTI to submit scores for an existing assignment (Created from within canvas)

[BenBelista]

We are tool that offers a canvas integration to our customers. We want to give our customers the ability to sync some data from us into already created grades in canvas. We want to do this because often, teachers already have their courses set up at the beginning of the semester, with the right categories etc, and we just want to get the grade data in the right place for them.

Problems we are having:

• we are able to easily create brand new assignments and line items and insert them into the canvas using the API or LTI integration
• We cannot seem to use the submissions LTI API on an assignment created through the canvas UI.
• After creating assignments from the canvas UI, no line items are created or accessible for those assignment using postman
• Without a line item, there is no way to create a score for a student for an existing assignment.

Is it expected that line items are not available for assignments created with inside canvas? Is there any other way to post scores for already existing assignments.