Currently, the only way that I know of to add an outcome to an assignment is to add a rubric that has an outcome to that assignment. This can be quite time consuming and sometimes, unnecessary. An assignment does not always need a rubric.
It would be ideal to have a +Outcome button next to the +Rubric button once an assignment has been created. This will track mastery of outcomes very easily and teachers can use that information when they are doing standards reporting and data analysis.