To load a JS file, you can use code like this in a plugin attached to the appropriate System Event (e.g, OnManagerPageBeforeRender):
$modx->regClientStartupScript('url/of/js/file');
That said, I think it's unlikely that you'll get a third-party editor to work in the modExt-based Manager without a lot of frustrating work.
Have you looked at the Redactor, TinyMCE, CKEditor, Ace, and CodeMirror extras (assuming that you mean MODX Revolution)?