The reality principle needs to be put in check here. First of all, using a converter is never the right thing to do. Why? The likelihood of you properly using the converter is slim to nil. My suggestion is to contact the owner of the home and let them know that there are outlets that do not provide grounds. This is a reasonable safety concern, and ask them if they'd be willing to swap rent for an upgrade of some or all the outlets. It may be illegal for them to rent the house under that condition. If they won't pay for the fix, foot the bill for an electrician to switch a few of them over to grounded outlets, it is probably a simple procedure because most old houses used metallic shielding and grounding is easily setup with these systems.