Thanks bheadrick for your answers, I totally missed that.. Sometimes it's easier then it looks. I got sidetrailed because i looked at the way how it worked and noticed that it writes to the wp_options table.
I did manage to classify it as "external" product_type at the automated import and it shows in the backend as an external/affiliate product_type BUT there is a big catch. It still does not show up in the frontend and the reason for this is that if i open the product and save it again, ONLY then it writes to the "wp_options" with the following values.
option_id option_name option_value autoload
764 _transient_wc_product_type_386 external yes
The number 386 is the postid from wp_posts. It does not automatically write these values to wp_options. Would it not be better to write this value to a postmeta field?
I think it's a bit strange for a post_type to write to the options table, that would be suitable for plugin's, theme's and the wordpress core?