Yes this is very possible :)
You can either use another Navigator outside the one you currently have and just use that for the modal (the router can handle multiple levels of navigation).
Or you can use a
WhileTrue that you just toggle on and off which has the modal inside it:
<Panel><!-- <- this is your modal panel, use AddingAnimation and RemovingAnimation to give it some animation when it appears -->
.. main app content ..
<!-- the modal can be toggled using for example the `Toggle` action:
<Toggle Target="modal" />