I get the error: "Cannot save file! Data format requires hardware type to be defined (.HarwareType) when saving as Tomato.
This happens if you are trying to convert settings from other firmware to Tomato format as this additional parameter is not specified elsewhere. But there is an easy way to get that one. Install Tomato on a router and do backup of the initial settings. When you open it in the WRT Settings, you will notice it has a field named .HardwareType. Remember value it has.
Reopen the original CFG file and (after removing read-only on toolbar) add .HardwareType field at the bottom with the number obtained from Tomato defaults. Now you will be able to save it in Tomato format.
PS: Do make backups of settings before - switching between firmware families can be tricky and backup will help if something goes wrong.