How can you add caffeine to small batches of soft drinks?

My best guess would be adding in the extract of some plant that contains caffeine. The African cola nut was traditionally used in soft drinks in the past. Coffee seems like and obvious choice, too. Maybe yerba mate?

is this legal in US???

Would this stuff work? It’s unflavored but it does have more than just caffeine

caffeine pills, just smash em up, and divy it out? You wouldn’t need to much to pack a punch, and they are pretty cheap.