It is not possible currently. You can add a feature request in the partner portal.
What might work, is generate a .url file (if the client is a windows machine) or a .html file with a redirect in it, and use the microflow download action to let the client download the file and open the page.
As Michel said it is not possible. However, you could call a java action from your microflow opening a url using a java action. Since java 1.6 the desktop class can be used to request the user specified browser and open a url. For more information see link
Edit: This solution will not work in most situations as Michel explains in his comment.
Is this still not possible?