Is there a way to add many A records at a go in F5 as opposed to one by one provided by GUI? I need to add 1300 A records. I can't load as if I am creating the zone a new as it will overwrite existing records.
The option in the GUI you are using is called ZoneRunner that is just a web interface for BIND.
As far as I know there is no tmsh (F5 CLI) for BIND, so the only option is to edit the BIND zone file directly.
Just use whatever tool you want to create all A record lines that matched the format used by BIND, and then paste the content to the BIND zone file.
Use the workaround in this solution that explains how to edit the zone directly via BIND: