Rus does not work that way if you're talking about the middle ages. If it's taking place later...well, you'd have to provide an outline first as to when you want to do it. But in the middle ages...
You'd need the entire Rurikid clan to die out, AND have the city assemblies and the nobles agree to Capetians taking their place. There's the incipient concerns about Roman vs. Byzantine Christianity to think about as well.
Finally, military institutions of Rus were on a personal loyalty basis by professional soldiers rather than territorially-defined feudal holdings. Whoever the Capet was, would he even have the prequisite personal army to be be able to get approved as the ruler?
Anyway, that's my two cents, but feel free to ask if you really do want to develop the scenario.