For Google Apps settings, there are two conditions respectively :
1. You can add Mail id only once:
You can add it in Google Apps under preferences in settings.
NOTE - Mail id is not editable (Once you add it can't be changed forever)
2. Enable and Disable Google Apps
You can enable and disable google apps anytime according to your requirement.
Gmail Signature - You can see the Gmail signature of the user for whom you have enabled google apps.
If Gmail signature is not added then -
Go to your Gmail account Settings > General Settings > Signature > Add Signature > Click on Save Changes button which is present at the bottom.