Execute add_post_meta during import

Hi,

I am trying to execute add_post_meta() from within a function call during the import of my XML file to create duplicate entries for a field meta key instead of overwriting it.
When I use my code within a init function, it all works fine, but when I use it during the import, the data is not getting written. Any idea what might go wrong here?

Context why I want to do this:
Instead of overwriting the data in a custom field for each row getting imported, I want to add another meta value with the same key to database, maybe there is a better way for doing this instead of using a custom function call?

Best regards,
Sebastian