I'm not sure of the specifics behind what you're looking to accomplish, but if you're just looking to display frames via a portlet, than creating the framesets in the html that gets injected into the portlet sounds like the way to go. Note that you will need to structure your frames/html to account for any width constrictions that take place in the portlet container/region (e.g. use of overflow to scroll horizontally, etc.). If the default layout in the portal is not sufficient, you can always tweak the theme css to adjust region width and position. Otherwise, you might run into a situation where the html fragment in the portlet will cause the layout on the page to look off.
Hope this helps.