Adding a link to wp_nav_menu()

Hello,

I’m trying to customize the wp_nav_menu – in current state the wp_nav_menu contains links to the pages and home page. And I want to add to specific position link to eg. category archieve. Is there any way like update functions.php or I have to make the menu manually (write code directly into template file).

I wonder if anybody knows if it is possible.

thanks
Lukas