No, customizing compendiums is not possible at the moment. It's a popular request/suggestion, you can read more about things surrounding it in this thread: <a href="https://app.roll20.net/forum/post/2940484/custom-compendium#post-8984613" rel="nofollow">https://app.roll20.net/forum/post/2940484/custom-compendium#post-8984613</a>
