Right-click on your ordinary collection name in the left hand panel and choose set as target collection – the plus symbol will appear.
Now go to your smart collection and whilst holding down the ctrl key click (highlight) each photo you wish to add to your new collection. Or use shift+click if all items are continuous. Then press the B key.
When finished you can delete the smart collection if no longer required.
Don’t forget to make the quick collection the target by right-clicking on it to reset the plus symbol.