Yes of course you can. You can use the hook_node_update_index() for this:
function YOURMODULE_node_update_index($node) {
// here is an example that fetches content from an url and appends it to
// to the data that will be indexed
$ch = curl_init();
curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
curl_setopt($ch, CURL_URL, "http://example.com/readme.txt");
$content = curl_exec($ch);
curl_close($ch);
// return the extra content that needs to be indexed for this node
return $content;
}
The above will of course only work if your custom pages are stored as nodes in Drupal. For instance if you have a node that is called contact
which shows content generated externally, then you need to fetch the same content in hook_node_update_index()
for this specific node.