if you just need to import the pages you can add them as "external pages". this allows you to set permissions, page properties, and add it to you navigation/sitemap but of course doesn't let you edit the page inside of sitefinity. I did this with some sites that were developed seperately but needed to be in our sitemap.
You could also wrap the content and code into user controls, then add those to the page. We did this for an application that we built. the pages had to live inside of sitefinity, but the programming was already done, and needed to be in user controls. Once again you don't get to edit the content in sitefinity, but you get to keep your existing code.
Finally, if you used any kind of content management system, you can easily import stuff like news/events/etc via the various managers, it works something like:
var mgr = new ContentManager("News");
var content = new mgr.CreateItem("text/html");
content.SetMetaData("Title", importedItem.Title)
...
mgr.SaveContent(content);
this isn't EXACTLY the code but it's something similar; you can use the api documentation to learn more.
If it's plain text, then unfortunately yes, you'll have to manually copy this into sitefinity. It is a bit of work, I did this with our hundreds of pages, but in the end it's worth it to have the cms benefits.
hope this was helpful!