Howdy Martin,
I would love to help you with this.
Unfortunately the CSV importer only supports “additional fields” that are created by our plugin, it does not support “custom fields” such as those used by ACF.
What you might be able to do is write your own miniature plugin/script that will fire off when a post is saved. Basically you could use the action save_post_tribe_event to alter an imported event when it gets saved. You could setup an additional field that imports the data for your custom field. When you import map the data to the additional field. In your script, check if that field is present, if it is save it to the custom field area (either as well or instead, the choice is yours). This might require a ~couple hours of dev time to write something up and test it to get it working right. It will certainly require a decent knowledge of PHP to write.
Does that all make sense? Will that work for you? Please let me know.
Cheers!
– Brook