There would simply be no point in doing this in 1918. Making peace with Russia (so that troops could be transferred to the west) was the priority. German (de facto) control of Ukraine had to be part of the settlement in the East because Germany needed Ukrainian grain to win the war. By contrast, weakening Russia's hold on Central Asia would be of no immediate benefit to Germany. If Germany ultimately wanted to get still more land for itself or its allies from Russia, the logical thing to do would be to wait until after war in the West was won, whereupon she could always add to her Brest-Litovsk demands...