Hi @DrAesthetic,
Once you have transferred the shares to Dhan, you will get an option to update your cost price as well as date of purchase. While we have a detailed post on CDSL to CDSL based transfer here, we will shortly update the article with the process to update the buy average.
However, trust us, the process is really simple and intuitive once the shares are credited to Dhan account. You can always reach out to help@dhan.co as well.