It is available, but only with non-computerized tickets (unless ZSSK stopped issuing those). While with 'standard' ones, system just doesn't allow to choose any point after last real RZD station, at least it used to be like this.
...at least they told me so one year ago.
btw, also interesting that ZSSK part in the above reference is counted using regular tariff (basically a sum) instead of more expensive standard ZSSK part (which would have been applied in case 'Ekonomnye zeny' were issued for like Moscow-Bratislava or so).
...don't know is that rule also valid for other railways issuing City Star tickets.
By the way, from yesterday ASU PP (UZ) is connected to EPA, so it is now technically possible do electronicaly book UZ domestic and international trans.
Regarding buying reservations, are they connected to a ticket or can I buy them separately
I come to a croatian counter to buy a sleeper reservation from Chop->Moscow but without having the CityStar ticket itself. Is this possible?
Then the clerk told me that he is unable to make reservation for the train with the numbers 307C and 340/350 Ч. He said, that there is no need for making reservation for such a train. Is he right?
Going to Chop by Bus/Taxi
What is cheaper: Buying city star ticket in Michalovce > 1500 km for two or is there the possibility to buy the tickets at the counter in Russia?