That's correct, citizen_elah.
You are free to configure BIND, but you should be aware that some modifications may interfere with the normal operations of the GTM system and/or the GUI record management utility. (The modifications mbayarena suggests are benign as long as they are appropriately coordinated with the GTM record management utility.)
Your F5 Support contract is intended to offer assistance resolving F5 software related issues. Support for custom BIND configuration does not fall under that umbrella except for the very specific default relationship between GTM and BIND. You may even be required to remove BIND customizations to troubleshoot GTM cases with F5 Support.
You might get lucky & get a Support rep who is knowledgeable & willing to help you out with a custom BIND configuration, but they are not required to do so.
/deb