attach one post from one post type to another post from another post type

Plugin Recommendations are considered off-topic here. One solution could be to add a metabox to your post like so:

Add to Functions.php

/** Add Post Metabox **/
function add_custom_meta_box() {
    add_meta_box(
        'pick_post', // $id
        'Pick Post', // $title 
        'show_custom_meta_box_pick_post', // $callback
        'agenda', // $page
        'side', // $context
        'high'); // $priority
}
add_action('add_meta_boxes', 'add_custom_meta_box');

/** Post Metabox Callback (show some stuff in box) **/
function show_custom_meta_box_pick_post() {
    $doelArr = get_posts( array( 'post_type' => 'doel', 'orderby' => 'title', 'order' => 'ASC', 'posts_per_page' => -1 ) );
    $meta_doel = get_post_meta($post->ID, '_doellist', true);
?>
    <input type="hidden" name="info_meta_box_nonce" value="<?php echo wp_create_nonce(basename(__FILE__)); ?>" />
    <strong>Associated Doel</strong>
        <br />
    <select id="selectdoel" name="_doellist">
        <option value="">Select Doel</option>
    <?php foreach($doelArr as $doel) : setup_postdata( $doel ) ?>
        <?php if($doel->ID == $meta_doel) : ?>
            <option value="<?php echo $doel->ID; ?>" selected="selected"><?php echo $doel->post_title; ?></option>
        <?php else : ?>
            <option value="<?php echo $doel->ID; ?>"><?php echo $doel->post_title; ?></option>
        <?php endif; ?>
    <?php endforeach; ?>
    </select>
}

/** Save Our Meta-data **/
function save_custom_meta($post_id) {
    global $post;

    // check autosave
    if ((defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) || (!current_user_can('edit_post', $post_id)))
        return $post_id;

    if($post->post_type == 'agenda')
    {
        // Doel Selection List Check
        if(isset($_POST['_doellist']) && $_POST['_doellist'] != '')
            update_post_meta($post_id, '_doellist', strip_tags($_POST['_doellist']));
        else
            delete_post_meta($post_id, '_doellist');
    }
}

What this does is create a small metabox with a dropdown of Doel posts – then saved the Doel Post ID as a meta value. You can get that meta-value (Doel Post ID) at any time by passing in the Agenda ID to get_post_meta() function.

$meta_doel = get_post_meta($post->ID, '_doellist', true);

Then you could get the entire Doel post like so:

$doel_post = get_post($meta_doel);