vahost on "Modify primary nav URLs on-the-fly by adding parameter"

I'm relatively new to creating filters, and consequently pretty weak in my understanding of how to create and use them. I'd appreciate a little guidance on how to create this one.

In this case, I want to filter the URLs of the primary navigation menu on-the-fly when the app detects certain circumstances.

My primary navigation items by default are not set with URLs that have a question mark and a myvar parameter at the end. They're just standard permalinks with the primary domain and slug.

But if the app detects a session variable that I've set elsewhere, I want to filter all the primary navigation items to add ?myvar=1 at the end.

Can someone help me accomplish this?