Not really plausible, but first step could be taking over of Liberia...
It's possible with the right POD and butterflies. What you need to do is get a USA more inclined for overseas expansion. Maybe one that loses out in the Mexican-American War. This POD is early enough to allow the USA to actually gain some decent footholds in Africa.
Jycee's timeline is the sort of situation that creates a plausible smaller America vs. a plausible larger Mexico, with neither side feeling particularly happy about it. The US that stems from it could eventually turn inwards, focussing on removing slavery and forcing the South to comply before a Civil War, or it could turn outwards and broaden the horizons of America's Manifest Destiny to something along the lines of 'rival the European empires' rather than conquering North America.
So this US jumps into the Scramble for Africa. In OTL, the USA was included in the Berlin Conference, but did not take much of a role in it. Perhaps in this ATL Berlin Conference, the USA has annexed Liberia (could also help subjugate the south - free the slaves, and send them away). But what else? The Congo River mouth wasn't grabbed by Belgium until the 1870s. ITTL, the Congo interior is much the same - disease-ridden and underexplored. American adventurers, working for the American Colonial Society (I think that's what it was called in OTL), might make this one of their targets.
The problem with Africa is that while other nations had clear motives (Britain's Cape-to-Cairo empire, Portugal's attempt at the Pink Map, France wanted a connection from Dakar to Djibouti), adding a new nation to the scramble (such as America or even the Netherlands) mixes up these motives. Who knows where exactly the US would colonise, but there is surely a way for it to happen.