Hi @PerS
Thanks for clarifying.
I know that this is possible using a filter on all WP WYSIWYG called ‘tiny_mce_before_init’, but this will be applied to all fields, not specific ACF fields…
I found this article on stack exchange which may be a good starting point:
http://wordpress.stackexchange.com/questions/3882/can-i-add-a-custom-format-to-the-format-option-in-the-text-panel
Good luck
Thanks
E