racism is irrelevant for colonialism. The European powers established colonies over plenty of people they considered to be inferior; you colonize places for the economic benefits to the mother country, not because you like the locals. Which is why there are a hell of a lot of differences between the European style colonialism and USA territory gains. The former were usually done on places with large native populations that were exploited for economic gain, and enforced with European troops and a veneer of European administration over it all. And that's the big difference with the USA's land grabs... they were done on areas that were pretty much empty of native inhabitants. The Americans never really exploited the native Americans economically, and the handful of Mexican inhabitants in the regions we took from Mexico scarcely compare to the big native populations of India and Asia. About the only remotely comparable land grab is Hawaii, which did have a fair sized native population, but even Hawaii went through the territory process instead of a real colonial process. Not that the USA's expansion was any less imperialistic (or brutal) than Europe's colonization of the old world, but the two aren't really comparable.
So, to answer the OP, how to get the USA into the colonial game. The old world is pretty much out, I'd think; the USA would scarcely want to tussle with the powers of Europe over any of it. So, the USA's colonialism would be limited to Central and South America. But the USA would have to be careful, as some of these places were still colonies of Europe, and Britain in particular had interests all over the area...