VoiceAttack can't natively trigger off that combination, but you can trigger off part of it, and check for the other part:
Start Loop While : Keyboard Key 'Left Alt' Is Pressed
Begin Device State Check : Left Mouse Button Is Pressed
Write '[Blue] Left Alt + Left Mouse detected' to log
Loop Break
End Condition
End Loop
Set the "When I press keys" option to "Left Alt"; Pressing and holding Left Alt will start the command, if you click the left mouse button while Left Alt is still pressed, anything before "Loop Break" will execute, and the command will stop checking for the combination and end(otherwise it'll execute continuously while Left Alt and the Left Mouse Button are down).