add_action in a function, is it possible?

If you want to find out whether you’re on a specific page, post, in a category archive, etc., then core has to offer Conditional Tags for this.

If you want to hook a js-script definition, then a better way to do this is the following:

// Assuming that you drop your js code into a separate file instead of just between <script>-tags

// 1st: Register during init hook
function add_my_script()
{
    wp_register_script( $name, $url, $dependency, filemtime( $path ), true )
}
add_action( 'init', 'add_my_script' );

// 2nd: Enqueue during enqueue hook
function enqueue_my_script()
{
    // Choose your conditions:
    // On Admin, check against "global $hook_suffix, $pagenow, $typenow"
    if ( 'post-new.php' !== $typenow )
        return;
    // On public pages, choose your conditions from the list of Conditional Tags
    // Example: Only on pages
    if ( ! is_page() )
        return;
    // ...Nothing to do on login

    // print the script to the screen
    wp_enqueue_script( $name );
}
// Choose where you need it
add_action( 'wp_enqueue_scripts', 'enqueue_my_script' );
add_action( 'admin_enqueue_scripts', 'enqueue_my_script' );
add_action( 'login_enqueue_scripts', 'enqueue_my_script' );

EDIT

You can sum up/collect all your little functions, that get hooked somewhere and put the add_action calls into a function that gets loaded on the first hook that’s available to themes: after_setup_theme. Important is that the child themes functions.php gets loaded first, then the parent themes and then the after_setup_theme hook. So you can let the possibility open to change things from inside your parent or child theme.

function theme_bootstrap_fn() 
{
    add_action( 'wp_head', 'js_func' );
    // other functiony things
}
add_action( 'after_setup_theme', 'theme_bootstrap_fn' );

// Define your function:
function js_func()
{
    echo "
        <script type="text/javascript>
            // do stuff
            // Add variables like this:
            {$some_var}
        </script>";
}

But to answer your question: Yes, you can call every global function inside another function.