Kiszka was also sent to diplomatic missions to Poland in 1492, 1495, and 1496. In August 1492, he represented Grand Duke Alexander at the Sejm of the Kingdom of Poland where Alexander's brother John I Albert was elected as King. In 1501, Kiszka supported the proposed Union of Mielnik which would have united Poland and Lithuania into a single state. The union was inspired by the continuing Muscovite–Lithuanian Wars, particularly the defeat in the Battle of Vedrosha, and Lithuanian desire to secure Polish military support against Russia. Kiszka also attended Sejms in Radom (1505) and Lublin (1506). After the death of Alexander, Lithuanian nobles elected his brother Sigismund as Grand Duke of Lithuania in September 1506. Together with Bishop Wojciech Radziwiłł and voivode Jan Zabrzeziński, Kiszka attended the Polish Sejm in Piotrków Trybunalski that elected Sigismund as King of Poland in December 1506. In general, Kiszka was a strong proponent of a closer union between Poland and Lithuania.