You could use <IFrame> but you are limited i nhow it's displayed and it may not look all that great with your current design. That of course is just personal opinion.
As far as grabbing other dynamic content, you may need access to the code on the site you want to get content from and then you would have to build your own script to fetch it and display it on your pages within your own design. Thisis the harder way to do it, but gives you more creative freedom in terms of presentation and layout.
I achieved this on a semi-private site I did for someone where the list of members and their details are changed on a domain other then my own. The script I use fetches the user details as they are updated or changed, on the exteranal site then added or removed from the site I have this information displayed on. When their name is clicked on, user details from the external site are also displayed on a separate page controled by me, in a very customisable way. Unfortuanely, this code is limited to only the external sites functions and would be useless anywhere else, so sharing it would be futile. It relies on code developed for the external site and also incorporates information from their own database, which is not hosted on the site domain I have built the site upon. I would show you the link, but it's not intended for public consumption and is limited to members of this group.
I'm not sure if I over explained myself or not, but I think I may have complicated the matter some.
I also forgot to mention that you will need to save your file as .php, not .html
Your link would end up looking like
http://www.yourdomain.com/customPage.php if you used the method explained and detailed in my earlier post. This could also be the reason why your page did not display. Generally, I just drop my html content into the XOOPS wrapper and worry about nothing else because this method is the fastest for me witout having to edit anything further. I also haven't tried using the include statement as you tried, so I have no knowledge on whether or not it will work, simply becaus e I never tried before.
I any case, here are a couple examples of what I have done wih the same wrapper you illustrusted:
http://www.stewdio.net/actd/about.php is just some html content with the XOOPS wrapper.
http://www.stewdio.net/guestbook is an entire 3rd party script I use. I simply edited the scripts code to output it's content in XOOPS. 3rd party scripts are touch and go, some work, some don't. Some need multiple pages wrapped in XOOPS, while others only need to have one file modified. It's truely experimental from a webmaster standpoint, but the results are outstanding when it works. I have done this successfully for over a dozen scripts.
As for <IFrame>, I don't like it because of the extra scroll bars and lack of control over how it is displayed. Again, not sure if thats what you wanted. I could be reading your defination of exeranl dynamic content wrong, so you will have to forgive me.