To add a new road to an existing junction, place the end of the road over the existing junction. Then choose "Add new junction" and tick "Enable all turns" and place a new junction over the top of the old one. Then apply any turn restrictions.
To add a new road to an existing junction, place the end of the road over the existing junction. Then choose "Add new junction" and tick "Enable all turns" and place a new junction over the top of the old one. Then apply any turn restrictions.