It isn't really designed for you to plug in .aspx pages in the menu, all the pages in the menu are served from /Default.aspx?pageid=x but we use url rewriting to make friendly urls, but the pages do not correspond to different .aspx pages on disk and the system is not really designed for that.
The best way to do it is to implement your functionality as .ascx user controls instead of as .aspx then you can plug them into the content system and add them to mojoPortal pages. See our Hello World developer tutorial for more.
You could also just add our built in iframe feature to a page and then show your .aspx page inside the iframe, but that would not be my favorite way of doing things.
Hope that helps,
Joe