Pretty much. A mainframe oscilloscope just has more flexibility with all it's plug in options that can be added. So as long as the mainframe oscilloscope has all the necessary modules installed, it'll do the same or more than an ordinary scope.
A mainframe oscilloscope has a backplane that the modules plug into, this is a common PCB usually with edge connectors or some type of standardized connection system for each of the modules that can be connected to it. Therefore a custom module is a module that you make yourself to add a feature you want to your oscilloscope.