As general rule, there is no way to do what you are describing. The exception to this is for media, for which you can integrate with the Music and Video hub. This would allow you to, for instance, download a movie and then play it in the same way as if it was part of the zune collection, even though it's in IsolatedStorage.
If you want to download office files from the internet you can simply link to them directly by opening them in a WebBrowserTask. The user would then be able to open the file in the relevant office program. So, if you did the follwoing, the file would be opened in Excel:
var wbt = new WebBrowserTask();
wbt.URL = "http://example.com/file.xlsx";
wbt.Show();
This would not allow you to store the file in IsolatedStorage and that may or may not be an issue for you.
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