You can define the available options using the choices argument as seen in the example below.
Example
new \Kirki\Field\Radio_Buttonset(
[
'settings' => 'radio_buttonset_setting',
'label' => esc_html__( 'Radio-Buttonset Control', 'kirki' ),
'section' => 'section_id',
'default' => 'red',
'priority' => 10,
'choices' => [
'red' => esc_html__( 'Red', 'kirki' ),
'green' => esc_html__( 'Green', 'kirki' ),
'blue' => esc_html__( 'Blue', 'kirki' ),
],
]
);