Resolved Confusion about export and import K2 items with extra fields

In the tutorials

https://rolandd.com/documentation/ro-csvi/k2/export-k2-extra-fields-and-import-as-joomla-custom-fields
https://rolandd.com/documentation/ro-csvi/migrate-k2-items-as-joomla-articles-using-rocsvi

It is assumed that the extrafields are visible and selectable when exporting K2 items. But even after Maintenance reset I don't see them in the available fields. When I create an K2 item export template and run it the extra fields and values are available in the exported file:

"title","catid","category_path","image","published","extra_fields"
"test","4","publicatie-verklaring-hebreeuwse-bijbel","","1","[{""id"":""1"",""value"":""undefined""},{""id"":""8"",""value"":""undefined""},{""id"":""7"",""value"":""undefined""},{""id"":""6"",""value"":""4""},{""id"":""4"",""value"":""undefined""},{""id"":""5"",""value"":""undefined""},{""id"":""3"",""value"":""undefined""},{""id"":""2"",""value"":""undefined""},{""id"":""9"",""value"":""undefined""},{""id"":""10"",""value"":[""in winkel"",""https:\/\/societashebraica.nl\/hikashop-menu-for-products-listing\/product\/"",""same""]},{""id"":""11"",""value"":""colors""}]"

which is also fine, but when trying to import it in my - still J3 - site with joomla content import templateI get an error in the preview:

Error
No column headers have been found in the file

The csv file is displaying correcty in Maintenance→ Analyser. I must be doing something wrong, maybe importing in a J3 site is not supported?

Additional question: Should I make a mapper file to connect the extrafield ids to the (new) Joomla Article fields that will recieve the data from the extra_fields, or how is that handled? The field mapper can as far I can see only handle direct fieldnames, maybe thats different in de newer version of of RO CSVI I will be running op J4?

Please point me in the right direction.

Greetings,
Nico