Hello,
There are few ways to achieve what you're looking for.
The most straight-forward is to use create_main_href() method. The basic syntax is:
$this->create_main_href($module_name, $function_name, $function_agruments);
This will create href that will enable user to switch the main module displayed in epesi. Be careful thou, as you need to make sure that this function provides navigation methods to get back to where user was.
If it's a problem, you can always create callback href, place navigation methods there and init+display the module you want in that callback.
Both are valid methods, depending a bit on the context.
Hope this helps,
Arek