On an SX900 the Pitch Bend 'range' is saved in a registration as part of the Style group for the Left voice part and the Voice group for the right voice parts.
So, if you memorise a registration with Style or Voice groups checked, the PB range will be saved as set at that time, and whenever you select such a registration, it will set the PB range accordingly.
There is no setting that can 'freeze' only the PB range in a registration. You can freeze the Style or Voice groups, but this will then also freeze the rest of the many parameters included in those groups.
When memorising a new registration that includes Style and/or Voice groups, you should ensure that the PB range is set as you want it for all voice parts before memorising it.
As far as existing registrations are concerned, I suspect that you will need to modify them one at a time by loading one, setting your required PB range and then memorising the setup back to the same button.
Murray Best has an excellent program for modifying registrations, but at this time the current version does not yet include the SX models, and in any event it does not allow you to change the PB Range, even on those models that are supported.
Regards
Ian