Sure. Why not?
There have been reports of merc sightings in the shadows of the upheavals in Ukraine from the beginning, and almost certain descriptions of the snipers in Kiev as mercenaries hired by Svoboda and the Right Sektor. Many observers have claimed they've been hearing unaccented English spoken among some of these shadowy forces, just before some Antimaidan dissenter is shot or otherwise incapacitated, and it is reported that the FBI and CIA -- among other US agencies -- are coordinating the crackdown on dissent and separatism in the East and throughout the country.
Given what has been noted so far in the efforts of the Kiev coup-regime to hold onto the East (Donbass) in spite of the people's defiance and refusal, there are no doubt death squads on the pattern of so many we've seen in the past) operating throughout Ukraine. They may be concentrated for now in the Donbass, but there's no reason to think their assignments and territory won't be expanded as resistance to the Kiev coup-regime's program of suffering and exploitation -- under the direction of the IMF, EU, USA, and NATO -- increases.
Mercs are an essential part of the program, especially when -- as in the case of Ukraine -- poorly paid and trained native "troops" refuse to fight their brothers and sisters.
Of course, media announcements of "The 400" or however many Academi (or are they Blackwater?) mercs there are in Ukraine look to be quite obvious propaganda to inspire terror among the refusniks and rebels. It's obvious they have no fear of the so-called "army" forces arrayed against them, as so many of the videos uploaded from the region testify. Perhaps they will fear the mercenaries, then, eh? The ones who have no qualms about liquidating the untermenschen wherever they encounter them.
If Ukraine isn't already swarming with international mercs -- they don't have to be Americans, do they, hmm? -- ready to shed plenty of native blood for the cause of the bankers and exploiters and financiers and oppressor-class, it soon will be, no?