"Can we...?"

The first thing that you must do before anything else when you start using a new device is to READ THE DATA SHEET!!!

You can download it in PDF format from the Atmel site.

On p80 you will find the Electrical Characteristics of the device - including the port pins.
This is what defines what you may and may not connect to the device: If your "real-world device" comes within the limits stated, then it's fine; otherwise it's not. Simple as that.

The only catch is that your "real-world device" must absolutely meet the specs at all times: not just most of the time; not just on a good day; but all of the time - including when it's switching on, and switching off, and under abnormal conditions, and under any transients caused by other systems (whether part of your equipment or not), and...
That's why it's usually sensible to include some sort of "buffer" between the micro and a "real-world device" - especially if that device is external to your system.