@seekingcanvas
I believe that you would need to make sure that no more attendance gets entered into RollCall to use that trick. Most external tools consider themselves authoritative for that assignment's grade and would override any manual entries you made.
Instead, I would recommend the approach that @NicoletteStaley recommended. You can make that extra assignment with no-submission and then explain how the grading works. Another benefit of the second attendance grade is that you can leave it blank until the attendance is done, or put a 0 in as soon as people drop below it -- your choice. With Roll Call Attendance, it would change the grade with each day you take attendance, so the student might have 75% grade there (overinflating their score) when they should have 0. That second assignment also allows the student to see their actual attendance percentage rather than just the binary 100 or 0. That gives them a warning as they go that they are getting close.
As Nicolette mentioned, for the Roll Call Attendance assignment, you should edit the assignment and check the box to not count it towards the final grade. Otherwise, you end up counting attendance twice. If you have a weighted gradebook, then another option is to create a category worth 0% of the grade and then move the Roll Call Attendance to that assignment group.