Populate a drop down list with post titles across a multisite network

You should be able to replace this section:

$posts = get_posts( 'numberposts=-1&post_status=publish&post_type=questionnaire' );

$choices = array();
$choices[] = array("text" => "Select Questionnaire", "value" => "");

foreach ( $posts as $post ) {
    $choices[] = array( 'text' => $post->post_title, 'value' => $post->post_title );
}

// update 'Select a Post' to whatever you'd like the instructive option to be

$field->choices = $choices;

with something like this:

$choices = array();
$sites = get_sites();
$choices[] = array("text" => "Select Questionnaire", "value" => "");
foreach ( $sites as $site ) {
    switch_to_blog( $site->blog_id );
    $posts = get_posts( 'numberposts=-1&post_status=publish&post_type=questionnaire' );
    foreach ( $posts as $post ) {
        $choices[] = array( 'text' => $post->post_title, 'value' => $post->post_title );
    }

    // update 'Select a Post' to whatever you'd like the instructive option to be

    restore_current_blog();
}
$field->choices = $choices;

By default, get_sites() returns up to 100 sites. You can filter this with various arguments.

Also, note that if you have a lot of sites in your Multisite network, or a lot of posts in your various sites, you should consider caching your results in a WordPress option or maybe site_option. switch_to_blog() can be expensive to run.

References

• get_sites()
• switch_to_blog()
• restore_current_blog()