You have a good feeling, sanitary and storm sewers are two separate systems and should be maintained that way, however, I’ve seen them accidently or purposely mixed up and spliced together many times. Also, older system may be designed that way (as one / combined) and include check valves (Palmer valves) that swing in-out preventing sewer water from contamination of the storm sewer section.
Another thing against such installation is a possibility of that internal roof drain contamination with debris, which would prevent proper venting of… is it just a powder room sink?
If it is just a sink, you can ask you local building department if they would allow “island venting” installation, also referred to as a loop vent.
Another option (if possible of course) is to extend that powder room plumbing fixture(s) vent above the ceiling and above the plumbing fixtures on the upper floor, and tie it to the vent stack at least 6in. above the flood rim of the highest fixture.
There’s also an Air Admittance Valve option (if it’s only the sink that requires venting), which is not approved by some jurisdictions, so you’d have to check with your local building department. The valve needs to be placed where it can be accessed for service / not inside the wall or ceiling cavity, etc.
There might be some other options to consider but I don’t know how is your plumbing system designed and distributed around that new powder room area… and how much or little damage to the walls and ceiling you’d allow.
Let me know