You already have the unit all coded I guess, because it's already in there. You're using ConvoyTruckArmedWithNuke I assume. Well then, you need to make a command button you can click on to build it. So open CommandButton.ini, and add this code anywhere in the file:
Code:
CommandButton Command_ConstructGLAConvoyTruck
Command = UNIT_BUILD
Object = ConvoyTruckArmedWithNuke
TextLabel = CONTROLBAR:ConstructGLAConvoyTruck
ButtonImage = SACLeopard
ButtonBorderType = BUILD ; Identifier for the User as to what kind of button this is
DescriptLabel = CONTROLBAR:ToolTipGLABuildConvoyTruck
End
Once that is done, simply open CommandSet.ini and add Command_ConstructGLAConvoyTruck to the GLA war factory.