a) Yes you can, and use it to pay to different packages.
However we do not allow you to add the same account number twice to the same service.
For example if you have already added your MTN number to MTN Airtime option, you cannot add it again. Now you will enjoy buying MTN airtime anytime using your Umba account
b) You can add many different accounts for example different airtime accounts, your data bundle accounts, your TV subscription account. In fact, Umba is a one stop financial solution app built just for you!