How would they do that? Russia has already invaded most of the east and south of Ukraine.
The UAF is not capable of flying over Russian territory. Not even an iota of chance to even approach Russia's airspace without getting intercepted or shot down.
The best they can do is to fire Tochka missiles and hope that it would hit somewhere because the CEP is not that good. And I believe they did it once, but if they resort to this pitiful tactic which has a high chance of not hitting its target and would most probably lead only to civilian casaulties, assuming that it won't get intercepted by S-300, S-400 and S-500, Russia will literally bomb them to the stone age without mercy. Russia is an extremely large country and Ukraine has only short range missiles if I'm not wrong. Meanwhile, Moscow is like 800 km far from Ukraine's borders. So, there's very little, if anything, that Ukraine can do.