When building your Integration , that is later used in tasks with each destination receiving 1 data type per call, you can use Workflow feature to modify the particular Data Type.
For example if the Data Type you are trying to sync is Contact List with additional attributes that are given from the Custom Fields Data Type ( another API call) you can create a Workflow that will modify your Contact List attribute schema adding the provided Custom Fields data as well.
Workflow option within the External API Template builder is displayed at the first step of the Template configuration.
Next you will be met with this screen where the created work flows for your Integration will be listed as you create them.
When creating a Workflow we are met with this screen where we set the following values:
Name - Name of the Workflow
Execution position - If you want to have multiple Workflows manipulating one Data Type you can specify the order in which they do so here by listing them (1-9, Optional)
Note: One Action can't be repeated multiple times for the same Data Type using Execution position.
Description - Description of the Workflow function. (Optional)
Listed Data Types for you to select from
Once you chose on which Data Type the modification is going to happen you will be prompted to pick an Action.
For this we need to consult the Integration documentation for the respective API Call (Documentation on that Data Type's handling) and, depending on the Action, continue as described in the articles linked below: