Only real way IMO is if you can secure the Pruth first. If the Pruth can be secured, successfully, then it can be a model for how you'd expand into the Ukraine.
As to being "Greek", I don't think that is plausible. I think you might get a Russian-speaking "Roman" peasantry, with a Greek and Russian speaking upper class, but it'll take a while for it to Hellenise, if ever. It is more likely to be a Russo-Roman identity that forms.
However, I think it has promise. Constantinople always needs grain, so if you can build a Pruth Model, that could be writ-large on the Dneiper upto Kiev as the river does freeze too much or too drastically. (In fact, it'd be cool to see Roman Icebreaker Galleys that keep the river clear in the worst years), then it can make a goodly some of money, and may be the preferred source, of grain for Constantinople and other parts of the Empire. The problem is I have no idea how easy it was to cross the Dnieper historically. If there aren't a lot of crossings, then great, a few forts and a river-fleet is great. The more crossings however, the more costly it becomes.
I'm quite curious to be fair as to what sort of benefit outside of good agricultural land this produces. Would it take the Cataphract and make it lighter and more suited for the Steppe? Would it use blocks of archers that can out-range steppe horse archers? I don't know all that much of how the Russians who fought the Steppe peoples and Won fought with prior to gunpowder.