I just tried to find out how to install additional Python packages for the standalone installation of Orange3.
I work on MacOS and want to use the "SQL Table" widget which needs pymysql installed. After installing all add-ons, including Prototypes, the said widget still tells me to "Please install a backend to this widget". This issue remains when pymysql is installed system wide - which by itself is less than ideal and apparently also not the way Orange is intended to work.
I was expecting some click and play install for packages similar to the one for add-ons or some prominently displayed information in the documentation which I failed to find (if it is there).
You can install any pip-installable package in the Add-on dialog, if you use the  "Add more..." button.
You can also install packages into the Orange app from a terminal if you run python or pip from the app. On my Mac, I would call /Applications/Orange3.app/Contents/MacOS/pip + any arguments. In fact, this is what the Add-on dialog does.
How to install Python packages for Orange3 standalone installation