First off, remember that you have a SCANNER and should learn how to use it to scan the various frequencies (not just what's been programmed, but any your scanner can pick up) and for the talkgroups how to use it's wildcard (by any other name) to get any that's in use by that system. That's how you'll be able to pick up stuff when things go bad. The known frequencies/talkgroups may be overloaded during a large incident and/or the system(s) may be damaged beyond use and no longer available. In a real disaster, the FCC can quickly assign new temporary frequencies to be used and agencies that arrive to help out during that emergency may get permission to use their own frequencies or system(s) (think a mobile site type operation) for communications. These may not be formal authorizations, but simply letters granting temporary access and not listed in the database (and that assumes that during this major disaster you'll have internet available to search anyway).
One example of this happening was during the deepwater horizon oil rig disaster where several states banned together to provide communications. There were portable sites from Louisiana sent to Alabama and Florida and others from Texas sent around the Louisiana coast. This may sound a bit weird that the LA sites were sent out of state while Texas sites were set up in LA, but this was because communication was needed to the east early and the existing LA sites could handle the initial rush while the rest of the changes were made to link everything up. Systems from Texas, Louisiana, Mississippi, Alabama, and Florida were linked together with many new talkgroups setup for those links to use. It was possible for an agency from one state to use a system from another state to talk to several command posts in other states to coordinate activity.