Well, he could. It just doesn't seem a likely war aim for Russia at the beginning of the war to defeat Germany and A-H, nab half of Galicia, then voluntarily surrender Poland and some other bits and bobs to form its own self-governing state.
Wasn't Congress Poland, self-governing for a while, the Russians would get the gratitude of the Poles, if they create a self-governing state within the Russian Empire, comprising all polish lands.