Hi @Marlen_Arias,
You can shape the retuned result using JavaScript code for the REST API data source: Import from REST API.
Here is another community topic on REST API result modification with JavaScript: eazyBI REST API data contains nest arrays.
An alternative is to use the integrated Jira data source, not REST API. Data import from integrated Jira data source supports issue custom field import (Data from Jira). This might be a more elegant solution.
If some 3rd party app creates the custom field, first, you may check if eazyBi has integration with that app and the field is already available (Data from Jira apps).
If the field is not available by default, you may enable and even process that custom field before import (Advanced settings for custom fields).
Best,
Zane / support@eazyBI.com