Roundcube skins define a number of named containers which can be used to add custom elements from plugins. Seek the skin templates for <roundcube:container ..> tags to locate them.
For the compose screen, there's the 'composeoptions' container which is probably the one you want. The plugin API wiki describes how to add elements to such containers:
Using JavaScript, this could look like this:
var menu = $('<select>')...
rcmail.add_element(menu, 'composeoptions');
You can also add elements to containers with PHP when the view template is rendered:
$menu = new html_select(array('name' => 'xxx'));
$menu->add(array(/* add options here */));
$this->api->add_content(
html::span('composeoption', html::label(null, $menu->show() . $this->gettext('menulabel'))),
'composeoptions'
);