Yes, Nannerl would be a nickname for
Anna. Nicknames ending in 'l' are (or were) common in
German; as well as Nannerl (which I've seen before, but it may have been the same person!) you get
Gretel / Gretl for
Margaret,
Liesel /
Liesl for
Elisabeth, and my own step-great-grandmother was
Rosa nn Rosel. There aren't many examples in the BtN database but those that are there demonstrate the pattern.
I hope that helps :-)
~Chrisell~
Proudly Australian
www.archaeochrisell.blogspot.com