Not possible. Slavic peoples are those who speak Slavic languages, one sub-group of the Indo-European languages.
10,000 years or so in the past, there were no speakers of these languges, because they didn't exist yet.
The most it might be possible to say is that Kennewick Man was a member of the group from which the Slavs descended.
But that far in the past, today's ethnic groups have very little relevance. For instance, assuming KW was descended from the ancestors of the Slavs. That same group is also the ancestor of the Germanic peoples, the Celtic peoples, and the Hindus. 10,000 years ago was long before these groups split apart.
So is KW related to Lenin or Gandhi?
Actually, he seems to be most closely related to the Ainu, an ancient people of the Japanese Islands that the Japanese practiced genocidal ethnic cleansing on for millenia.