First, why do you want to do this - what are you trying to accomplish / avoid?
Second, both isoparrafins and aromatics have higher octane values. Each batch of gasoline is blended to balance production of all various components vs all specifications required, so no, simply using higher octane fuel does not guarantee increased isoparrafins.
Third, maybe you can purchase isooctane - ask friends in your areas that are into high HP racing. That and maybe isoheptane would be the only isoparrafins I can think of that purchasing seperately and adding to a gasoline mixture might be in any way practical to a point. But to what point?