what exactly would be exported ? where in the frontend would that be available ?
The export-feature could be on for example on these pages:
applications.php?groupid=6&hostid=10058
or
items.php?hostid=10058&filter_set=1&filter_application=My+Custom+App
The imprort again could be here:
applications.php?form=update&applicationid=334
Or then the import could create a new application if that does not exist when importing the custom app.
The export should contain also all linked triggers etc. The import again should be able to add new triggers based on the manual edit of the xml-file. The correctness of the xml-file should definitively be verified by the user... messing with xml-files, new trapper-keys, triggers etc should be for the more advanced user 
Coming back to this, is there anything that is not handled by templates and/or LLD?