yes, you can absolutely mix DVD and downloads.
When you buy the DVD version (make sure the package is sealed), you'll also be given a code. The code is the important part. You will need to register that through Origin and then you can either install from DVD or download through Origin.
I don't work or have any association with EA. I give advice to the best of my knowledge and cannot be held responsible for any damage done to your computer/game.
Please only contact me via PM when asked to do so.