Alright, nevermind, I found a solution, thanks to this topic :
http://support.advancedcustomfields.com/forums/topic/adding-fields-using-functions-php/
=> In ACF menu, there is an export feature I had never noticed. (my bad).
I used it to get PHP code for my fields group (options page), pasted it into functions.php and VOILA : it works as expected.
Only tiny bug I noticed is it doesn’t handle accentuated characters
Sorry for the noob question.
… and glad to see once again how great ACF is !
Cheers,
Baptiste
