It is not so easy to change buffer. You must change descriptor file, not just length(If I understand usb correctly).

You can send one letter and modifier key at time.

Use this

Code:
 For i = 0 To 20	
                lookup i,[KEY_V, KEY_A, KEY_L, KEY_U, KEY_E, KEY_ENTER, 0], USBBufferOut[2]
                call DoUsbOut
                IF USBBufferOut[2]=0 THEN EXIT
            Next i
You can use array
Code:
 For i = 0 To 20	
                  USBBufferOut[2]= MyArray[i]
                call DoUsbOut
                IF USBBufferOut[2]=0 THEN EXIT
            Next i