You can do it, but its a big process.
1. Duplicate your existing TFT MPQ file.
2. Open the MPQ with an MPQ opening tool.
3. Extract all the campaign maps from the MPQ.
4. Edit them as you like (new heroes etc.)
5. Copy all the files from the original MPQ to a folder.
6. Add the replacement maps into the folder.
7. Make the new MPQ with an MPQ tool and put it into the WC3 folder (replacing the original MPQ).