Use the `wcpay_multi_currency_available_currencies` filter to add another currency to WooCommerce Payments Multi-Currencies.

functions.php

<?php // do not copy this line

add_filter(
	'wcpay_multi_currency_available_currencies',
	function ( $currencies ) {
		$currencies[] = 'BTC';
		sort( $currencies );
		return array_unique( $currencies );
	}
);