You certainly could do that...but there are things that should be considered. Like Josh said, there's always a risk of bringing pests or even just eggs/larvae from pests back into your grow space. Then there is the difference of light between your 400w HPS and the sun. I think it could be done, but I'm not sure how much the plant would like recieving two completely different measures of light everyday. Just my 2 cents though, good luck.