Get import_id

I’m sorry but the import_id is currently not broadcasted by the pmxi_saved_post action. You could use a custom field to differentiate the imports. Simply set them up on the import template and query the field based on the post_id sent by the action.

Solution: use $_GET['ID']

Just to note that that is a solution that works for now, but as it is not supported it may break at any time. Accessing a value stored on a custom field is a consistent and supported way of differentiating imports.

What it is name of custom field?

It would be your own custom field, so you can name it whatever you want.

To be clear this is a custom field you setup within the Custom Fields section of the import with a fixed value to identify your import. That custom field will be created for all posts that are imported through that import.