You'd probably be better off just converting it to an epub, and loading the epub to your Opus.
Select the zipped book in Calibre, then hit the "Convert EBook" button up in your button bar. Make sure the "Output Format" dropdown (in the upper-right corner of the convert screen) says epub, then hit OK. It'll chew on it (probably not more than a few seconds depending on your machine), and you'll have an additional format for the book. View the resultant epub (either click the number for the item, which defaults to displaying the epub if it can find it, or edit the individual metadata for the book and double click the epub entry in the list) to make certain it looks ok.
Assuming so, just send it to your opus. Calibre will send the epub, instead of the zip, and you should be able to find it there.