Registering as a family member

[WithLotsOfSprinkles0]

My best friend is due to become a mother to an adopted baby.

She has no close family members and says she is putting me down as a family member and auntie to her baby. I'm thrilled of course, is there any way to make it official?

I assume she means including you in her description of her support network, otherwise I don't really understand what you mean by putting you down.
When I grew up it was very common for your mum's friends, and often neighbours
to be known as Auntie as a sign of respect.
If she wants her child to call you auntie sprinkles I think that's a lovely thought and shows the childs the strength of your friendship.
It is an 'honorary' name though, I can't see how it could be made official in any way. Unless she wanted you to be a godparent?