You can create your own custom fields and add them to the Report Designer palette. To do that, you have to:
For registering a custom field, say MyField, add your control to the <customfields>
section in the C1ReportDesigner.exe.settings file as follows:
<customfields>
<!-- THIS LINE ADDS A NEW FIELD TO THE DESIGNER -->
<item value="MyCustomFieldAssembly;MyCustomFieldAssembly.MyField" />
</customfields>
Note that the code above assumes that your field is called "MyField" and it can be found in the assembly called "MyCustomFieldAssembly".
Following are built-in standard custom fields that are loaded in the C1ReportDesigner application by default: