Hey Faaktoo,

What I think you are looking for are called modifier keys. The modifier keys are keys such as shift, ctrl, alt and so on. They are set by setting the value in USBbuffer[0].

Modifier Key Bits
0 LEFT CTRL
1 LEFT SHIFT
2 LEFT ALT
3 LEFT GUI
4 RIGHT CTRL
5 RIGHT SHIFT
6 RIGHT ALT
7 RIGHT GUI

For example,

USBbuffer[0] = 00000010b

would register as having the left shift button pressed. I haven't actually tested this, and it's been a while since I played with my pics so hopefully this helps you out.