“In addition to the
member states of the union, the countries part of the
European Free Trade Association (EFTA), i.e.
Liechtenstein,
Norway,
Switzerland and
Iceland, have been granted participation under Article 66 of the Basic Regulation and are members of the Management Board without voting rights. There are also numerous working relationships with other authorities.”
I doubt any country, other than Member States, could participate with voting rights. (Re)joining EFTA would also pose problems for the U.K., but they likely would be welcome to.
Of course the Basic Regulation could be amended to allow this, but that would likely be another drawn out process.