You will need the source to change the URLs for the routes. The custom routes are located in Presentation\Nop.Web\Infrastructure\RouteProvider.cs. Change the second parameter in the MapRoute method for the route you want to change.
This refers to nopCommerce 2.00 and you will need to recompile.
Would it be possible somehow to render the ContactUs.cshtml into the TopicDetails.cshtml by using an if sentence? Like:
if (currentURL == "MyPage") {
RenderPage("/Views/Common/ContactUs.cshtml"); <--- This obviously doesn't work as it doesn't incorporate the ContactUs model.... But still would be nice to be able to do something like that.