When Cassius met with US troops to get weapons for his rebels, he straight up told them that the rebels were fighting for themselves not the US and that the revolution would come to the US in time as well.
"Cassius nodded. 'It's right, but we make dis revolution fo' our ownselves, not fo' you Yankees. Like I tol' you' sergeant here, one fine day you gits yo' own revolution."'
-American Front, Chapter XVI
Given Cassius is our viewpoint red leader, I think that suggests that the reds had no love for the United States and saw it as just a convenient, temporary ally and fully intended to establish their own communist nation.
I've got an outline for a TL-191 scenario where the Black Communists do a 'Long March' and concentrate in Mississippi/Alabama, where their local majority makes them much more effective at holding land against the CS Army and they do enough damage for Custer to break through in Tennessee in 1915, giving them a secure line of supply to the US and effectively cutting the CS in half. I think in that scenario, the US annexes Tennessee in addition to Kentucky, and Mississippi becomes a Red Black Republic (which the US maintains because it cuts the CS in half). Ultimately still doing its own thing, but given a fairly long leash and lots of supplies from the US because of how much their existence weakens the CS.
But I don't think a more complete annexation in GWI is possible, because by 1914, the prevailing sentiment I get in TL-191 is that the US just wants to make the CS
suffer, not share a country with them. Annexation became feasible in 1944 for a few reasons that don't apply in the earlier war:
Featherston had shown that Confederate revanchism could still be an existential threat to the US.
Superbombs made it a matter of survival for the US to secure control over the whole continent.
The US army had already overrun most of the CS in its effort to win anyway.
For the US to annex the CS in GWI, you'd have to create a scenario where the US overruns so much of the CS that mopping up the rest is politically acceptable and militarily trivial. No armistice short of Richmond, but US troops at least in Atlanta, Richmond, and New Orleans. Barrel Roll all the way to Atlanta at minimum, though the rugged terrain there would start favoring the defender.