Is it possible to add levels to the campaign in a seamless way?

For instance, if I wanted to push level 1-3 back and make it level 1-4, and then create a new level and assign it to be 1-3. Basically expanding the current campaign.

Also if I do this, could it be played through the normal campaign method.... or could it only accessed through the modding menu?

Lastly, could I add a new area to the map and add levels to it for the campaign?

thanks for reading,