My workshop sub-panel is powered from two dedicated 110V 40A breakers off the main panel
A subpanel is normally powered from a double pole breaker, and not two separate breakers.
Two individual circuit breakers is acceptable (and meets code) for 240 circuits (and to feed sub panels) as long as there is a tie bar linking the two breakers. This ensures that the two breakers are on opposite legs of the service and also it will make you shut off both if needed to.
Yes, a 240 volt circuit can be added to the sub panel. You say the tool (240v) is 7.5 amps. However, will it trip the sub panel main breaker? We don't know what you will be running off the other 15amp breakers (at the same time) but I would think you would not have everything running at the same time. Normally a "load calculation" is done to see if the service (the sub panel in this case) will accept the load of 7.5amps but I would think it would. You would have to be running other tools at full load amp rating of the receptacles to trip the 40amp breaker in the main panel feeding the sub panel.
What 240v tool are you going to be using on the workshop circuit you are adding so you know the proper receptacle to install on the circuit. Does the tool/device have a name plate on it? If so can you take a pic of it and post it this way we can guide you as to what you need to install for a breaker, gauge wire/cable and the proper receptacle.
