I was still confused about how to generate additional addresses and the keys for these addresses. Then I read this post: Create a signing key from an address in Daedalus wallet (Resolved) - #7 by bwbush
Explaining:
The payment keys in the wallet are generated by the series 1852H/1815H/0H/0/0, 1852H/1815H/0H/0/1, 1852H/1815H/0H/0/2, …
The change keys in the wallet are generated by the series 1852H/1815H/0H/1/0, 1852H/1815H/0H/1/1, 1852H/1815H/0H/1/2, …