StripyWitch wrote...
If I do it that way will I still be able to load a save and characters from the original campaign?
Characters yes, saves not easily.
Save files contain a lot of material from the original module, so the chances are that loading a saved game will over-write any changes you subsequently make.
With custom modules, you can work around this by putting changes in a hak, which takes priority over the saved game content. IIRC this is not an option for the OC.
If you know what you're doing, you can modify the contents of a saved game by editing the container file, but I'd regard that as advanced modding.