Hello,
The 16 channels correspond to IEEE 802.15.4 PHY and MAC layers specification. Since the radio embedded in the SiP is 802.15.4 (RF4CE, ZIgbee, etc) compliant, only those channels are to be used as defined by the IEEE. Why would you like to change them anyway?
Also, what do you need to modify from the MAC layer? Maximum packet length is also defined by the IEEE but you can always send less bytes if necessary. There is no need to modify the MAC layer at all for that. You could simply set the packet length in software and send the bytes needed.
I hope this helps. Have a nice day.
Angel