Matt --
I realize that you already posted an answer, but i wanted to ask why you're needing to add a SIS ID in the first place? You shouldn't need to set a SIS ID in order to retrieve any enrollments from a section.
When dealing with Canvas via the API, you can use either the canvas internal ID to specify which object you're working with... or you can use the SIS ID that you've set yourself.
For example, this is the endpoint when getting a section:
Using the internal Canvas-assigned ID:
$endpoint = $canvasUrl . '/api/v1/sections/' . $id;
Using the SIS SECTION ID assigned by us:
$endpoint = $canvasUrl. '/api/v1/sections/sis_section_id:' . $id;
So, you should be able to eliminate the first step where you're assigning a SIS ID. Just grab the enrollments from the section directly, and then do your operations like before.
(Unless im missing a reason for why you need a SIS ID in the first place, then ignore this whole reply 🙂 )