You cant without editing core files but you can name it any way you want so you can call it
from you plugin:
if ( function_exists( 'add_image_size' ) ) {
add_image_size( 'my-plugin-thumb', 220, 180, true );
}
and call it
if ( has_post_thumbnail() ) {
the_post_thumbnail( 'my-plugin-thumb' );
}
it’s not perfect but its something.