To tag our custom field as project-importable, we need to implement the following interface:

com.atlassian.jira.imports.project.customfield.ProjectImportableCustomField

You will have to then implement the following method:

ProjectCustomFieldImporter getProjectImporter();

There are existing implementations for the ProjectCustomFieldImporter class, such as the SelectCustomFieldImporter class, which we can reuse. It is in this class that we check whether the value getting imported is a valid value or not.

For example, in the case of a select field, we need to make sure that the value being imported is a valid option configured in the custom field on the target system. It is entirely up to the users to implement the various rules at this stage.

See the Javadocs at http://docs.atlassian.com/jira/latest/com/atlassian/jira/imports/project/customfield/ProjectCustomFieldImporter.html for more details on doing custom ProjectCustomFieldImporter implementations.