How to change the volume of my computer by assigning volume control functions to Shift+F7 and keep the normal functions of F7 when not holding the shift key?

With key transformation you can assign volume down and up to a key sequence ( such as shift+F7) easily. Because the key code of volume up and down are #175 and #174. So you can assign the key code to a key set. Do as following screen-shots:

Shift+F7 to Volume down:

Shift+F8 to Volume up:

Click here to enter homepage of Key Transformation