I was able to implement my own custom action that does this specifically for the sub-assigned field. The UI only allows the selection of a single field to perform an action on, so I used that as the source of where I want to copy a value from. This action will always copy it to the subassigned field but could be modified to alter other fields:
add_filter( 'dt_workflows_custom_actions', function ( $actions ) {
            $actions[] = (object) [
                'id'        => 'mtm_copy_to_subassigned',
                'name'      => 'Copy Relation Field to Sub-assigned',
                'displayed' => true // Within admin workflow builder view?
            ];
            return $actions;
        }, 10, 1 );
add_action( 'mtm_copy_to_subassigned', function ( $post, $field, $value ) {
	if ( !empty( $post ) && $post['post_type'] === 'contacts' ) {
		$updated_post = [];
		$updated_post['subassigned']['values'] = [];
		if ( !empty( $post[$field] ) ) {                
			foreach ( $post[$field] as $connection ) {
				$updated_post['subassigned']['values'][] = [
					'value' => $connection['ID'],
				];
			}
		}
		// Assuming we have updated fields, proceed with post update!
		if ( !empty( $updated_post['subassigned']['values'] ) ) {
			$updated_post['subassigned']['force_values'] = true;
			DT_Posts::update_post($post['post_type'], $post['ID'], $updated_post, false, false);
		}
	}
}, 10, 3 );