Also, you can 'override' templateID bindings with your own templateid bindings in a separate template binding file, which you then place above the ones which you want to override on tab 2 of the code generator configuration dialog.
This means you can create new templatebindings files and just bind the templates (copies of originals) which you want to modify.
If you want to add template code, look into using include templates. In general you just need to add templates, never alter ones.
Also, check whether you can avoid generating new code at all by creating partial classes of for example CommonEntityBase or DataAccessAdapter for example.