You can export a Custom Field by its id
Step-by-step guide
Custom Field Name: Age
Custom Field id : 123456
${Age[123456]}
Tip: To find the custom field id go to Issues - Custom Fields - Choose the custom field that you want and press edit:
Link for related documentation:
https://confluence.xpand-it.com/display/jiraxporter510/Mappings#Mappings-CustomFieldMappings