Eugene's answer above is excellent, and outlines some key features in Cubase that you should find useful in this task.
I'd just like to add that you can adjust the velocity of the MIDI events also - this effectively changes the volume, but it is more akin to the force with which the note is struck (ie pressing a piano key softly would equate to low velocity).
The Logical Editor can be used to change the velocity of MIDI notes, I found a tutorial here which looks like it might help you:
[TUTORIAL] How to humanise a midi (drum) track in Cubase
MIDI Quantizing and Humanizing Functions in Cubase
When you open the MIDI clip window, the velocity can be selected in the automation strip at the bottom, and you can use various tools to draw velocity in there. You may have already experimented with that, but I'm just mentioning it because I only found the additional tools after I'd been using Cubase for a while.
An another way to easily randomize the velocity in Cubase is to open up the midi stem in Cubase (double click it in the DAW). Then select the area you want to affect, or leave it all unselected. From the main menu, click on MIDI -> Logical Presets -> standart set 1 -> random velocity (60 to 100)