In Project, use File > Save As > XML Format (*.xml), then choose that file. It is checked before anything is saved. .mpp files are not read.
Your file is not kept. For security, HALEPM reads the uploaded file once to import the plan, then discards it. Keep your own copy.
Each work resource is matched to the pool by email, then by its exact name when only one active resource has it.
Each field's values are stored in the HALEPM task field with the same name. Fields are not created by the import.