In the XML mappings for the export and import( DomToXML_ExcelTemplate & XMLToDom_ExcelTemplate) the input mask is missing.
If you want this to be added, it might be best to contact Mendix, because its there module and ask them to add the field(maybe create a support ticket for this(https://support.mendix.com/hc/en-us)
As a workaround you could create a message definition(starting from the importer template object) and from this derive the correct export and import templates and use those.