Yes you can as long as extending that file to 30 extra days doesn't exceed the maximum retention policy for the SmartLock domain.  You can do this in the same way you would with NTAP, that is, set the atime to the new expiration time.  When SmartLock sees an atime in the future (which in normal situations should not happen) it will treat that as the expiration time for the file.  So from NFS, you could use something like 'touch -at file' or if you have a script that can change file metadata via system calls that would work too.  There's an equivalent thing for Windows, but I don't know it off the top of my head, but it would work similarly.  Hope this helps
Is it possible to perform Isilon SmartLock retention/hold related operation via an API? Something like Retention Centera API?
Isilon SmartLock Retention API?