How do I create a global MongoDB Admin account to see all Dbs?
You created the user correctly, but you missed a parameter when connecting to mongod.

By default, the authentication process assumes that the user is stored in the database you connect to, which in your case simply isn't true.

You need to specify the autheticationDatabase option:

mongo yourMongodIpAddress:27000/targetDb \
--authenticationDatabase admin -u DBASupport -p yourPw