Hi,
I’m trying to query a custom post type and then populate a ACF Dropdown box. Is this possible?

Yes, here’s an example:
The code below adds the roster for a team based on certain criteria.
function add_away_rosters_select($field){
$sports_cat = array('boys-basketball','girls-basketball','boys-hockey','girls-hockey','baseball','softball','boys-lacrosse','girls-lacrosse','boys-tennis','girls-tennis');
$cat = get_the_category();
$sport = $cat[0]->slug;
$field['choices'] = array();
if( in_array($sport,$sports_cat) ){
$away = get_field('away_team_2')->ID;
$args_1['post_type'] = 'roster';
$args_1['post_status'] = 'publish';
$args_1['order_by'] = 'title';
$args_1['order'] = 'ASC';
$args_1['meta_key'] = 'school_team';
$args_1['meta_value'] = $away;
$args_1['category_name'] = $sport;
$args_1['posts_per_page'] = -1;
$away_players = get_posts($args_1);
$away_roster = array();
for($i = 0; $i < count($away_players); $i++){
$away_player = get_field('last_name',$away_players[$i]->ID).", ".get_field('first_name',$away_players[$i]->ID);
$away_player_ID = $away_players[$i]->ID;
$away_roster[$away_player_ID] = $away_player;
}
asort($away_roster);
$field['choices'] = $away_roster;
}
return $field;
}
// changes the choices to players from the away rosters
add_filter('acf/load_field/key=field_58dadbb26c60a', 'add_away_rosters_select');