Have not got MS-SQL available as a policy type so did the following as Oracle which I hoped would behave similarly (but it may do this irrespective of the policy type anyway ***EDIT*** it does - just tried for MS-Windows-NT & Standard policy types).
Created a new Policy, skipped the Attributes setting & dove straight in to the Schedules & low & behold I got the same thing - the override was greyed out.
Had another look at the attributes tab & Policy Storage Unit was set to "Any Available". Set this to a specific STU (which is what I would normally do anyway, just skipped it initially for this test) & then found that I was able to override the volume pool in the schedule.
Give it a try (i.e. ensure Storage Unit is not set to "Any Available") & let us know.
I tried the same thing earlier, and for some reason, the results I got were different than Andy's. I skipped straight to the Schedules tab as well, and I am able to mark the checkbox and choose "Override policy volume pool:" I was able to do this for the MS-SQL policy I set up.
Make sure that you do not have "Multiple copies" selected, as that was the only option that prevented me from overriding the policy volume pool (at least that I could find so far).
Cannot change "Override Policy Volume Pool" in a SQL policy
This is by design of netbackup.
You can reference the following document item: http://support.veritas.com/docs/290212
VERITAS NetBackup (tm) 6.5 for Microsoft SQL Server System Administrator's Guide Page-30 Tips
If you put multiple batch files in the same MS-SQL-Server policy, they will execute during each Automatic Backup session for that MS-SQL-Server policy. If you have a variety of SQL Server backup operations that you wish to have executed on different schedules, then consider creating multiple MS-SQL-Server policies with differing Automatic Backup schedules; and then assign each batch file to the policy that uses the desired Automatic Backup schedule.