Hello Mueller ,
See http://www.nxp.com/documents/user_manual/UM10858.pdf , in the document, you will get detailed information on how to configure PN7462's registers.
See http://www.nxp.com/documents/user_manual/UM10913.pdf ,in the document, you will get detailed information on how to use APIs we provided.
So if you want to design your own API, you will have to do following steps:
(1)Knowing how to configure register through UM10858.pdf
(2)Knowing how to use PN7462's API to write/read registers.
(3)Using PN7462's API to write your own API.
Hope above information can be helpful for you!