In addition to use pre defined connectors, your also able to use custom code code inside of a Card or Micro-App.
Custom Code can be added manually or by using the Data & Actions option "Add Data Source" -> Select "Custom Service" as a Datasource.
For both ways, manually or by using the wizard, you'll have a class preferably in the Services or DataTypes folder, which uses the attribute "OneService" to get recognized by the designer.
The same goes for custom datatypes. They also can be coded. It's required to use the DataContract and DataMember attribute above the given class and property defintion.