When importing using JIRA Redmine importer, it will always map to a new text custom field.

When you perform an import from Redmine to JIRA using JIRA Redmine Importer plugin: https://marketplace.atlassian.com/plugins/com.atlassian.jira.plugins.jira-importers-redmine-plugin
it will always create a new custom field with the type text, even if you create the field previously and map it to the correspondent field.
So if you have a multi select list field for example, it will map only the selected value from redmine to a new text custom field.