Is it possible for a Byakugan user to awaken the Rinnegan?
Unlikely.

While it is true that Kakashi originally said that the Uchiha clan (And the Sharingan) is a branch off the Hyuuga clan (and the Byakugan), it was later revealed that it wasn't the case And even though Kaguya Otsutsuki was said to have the Byakugan, it is currently presumed that the two bloodlines are separate and there's no actual connection between the two eye techniques..

No Byakugan upgrades were ever discussed/revealed, like you said, the Hyuuga specific techniques are only the Byakugan's potential being utilized.

SPOILER!


   Moreover, it was revealed that the way to awaken the Rinnegan would be to combine Indra's and Ashura's chakra, like Madara (who had Indra's chakra) implanted the flesh of Hashirama (who had Ashura's chakra), to bring out the Sage of Six Paths' chakra.

 So theoretically, if Indra were to posses a Hyuuga clan member, and the identity of Ashura would be known, it may be possible. That, however, never happened and is not likely to happen in the future.