Is it possible to have civilian graduate level education (doctorate) added to my ERB if the degree is still in progress?

ERB/ORB requires transcript. Best thing is to graduate and have complete transcript added to record.

Last I knew, you need to have completed the course before getting it added. Did you ask your S1 this?

AR 600-8-104, Required document List, has been published to standardize rules for uploading transcripts to Soldier's iPERMS record (AMHRR/OMPF).

Transcript Requirements for Updating Army Personnel Records, the transcript must bear the following information:
1. Educational institution seal, watermark, or letterhead.
2. If conferring degree is listed, need type of degree (e.g. associates, bachelors, masters) and date the degree was awarded.
3. Educational institution official's stamp or signature.

The MILPER can be viewed at https://www.hrc.army.mil/content/18197

That came from HRC on their updating board records.

You can't add a degree you don't have yet. You can only add degrees you have received and have a transcript for.