Indeed, very problematic. It's not really so much a matter of logistics. Cossacks dominated the Siberian trade, including shipping along the north coast. They could cross the Bering Strait with no difficulty at all. The problem liessomewhere between motiovation and population. The European population of Siberia was tiny, and concentrated in the west. A Cossaack group coming to North Americamay well be tempted to stay and expoloit the resources and natives - it's what they did at home - but they would do it in the usual pattern. That means building trade posts and levying taxes in kind. It also means they get nothing they couldn't get closer to home, which makes America comparatively unattractive. Without a major pull factor, you won't get enough people thereto found anything like a host. And with that pull factor, you get tremendous butterflies.
A vague idea: The Russians lay claim to the California coast. Explorers, accompanied by cossack guides, range down the shore and a few Spanish settlements have the new facts of life explained to them. There is a crisis, a treaty, and the coast cis-montane belongs to the Czarina. So far, no big deal - then some lucky guy strikes gold, and Siberians stream there in their - well, high hundreds would be a major exodus. This is around the time of the Pugachev rising, and the government chooses to resettle disloyal cosack clans. America looks good. They arrive as military peasants, but many of them gravitate to the gold fields. By now, the California gold mining industry is a officially Russian, but swamped with non-Russian diggers, some coureurs from New France and especially people New Spain. The viceroy extends his protection againstthe ourtrages perpetrated by the Russian government, there is a flashpoint, and this time Spain gets serious. The Russian squadron in the Pacific is taken, a cavalry column arrives from Central Mexico, and Russian government ends in a brief, desultory fight where both sides mostly combat rampant desertion. Russia has to concede California because it cannot supply it. The cossacks now find themselves stuck as subjects of a hostile, Catholic, rather high-handed government that immediately sets about monopolising gold mining and turfing out the foreigners. As aresult, they decide to leave. The Spanish government has neither the means nor the interest to hold them (labourers from the Philippines and Fujian are cheaper and more docile), so we see a few thousand Cossack voortrekkers making their way northeast across the Rockies by 1780. They know what they'll find - some of them havealready beenthere - and they are confident they can carve out a better life for themselves among the native tribes in the northern plains and foothills than they would get under the Spanish yoke.