On 14 January 1940 Rökker was transferred to the "Luft-Nachschub-Kompanie 5" (5th Aerial Replacement Company) stationed in Gütersloh. From 4 July 1940, he was stationed at Berlin-Gatow with the "Schülerkompanie Flugzeugführer-schule" (School Company at Advanced Flying School). Rökker was promoted to "Gefreiter" on 1 October 1940. He then completed advanced training at a flying school near Magdeburg from 20 March—15 August 1941. Rökker attended "Blindflugschule" 5 in Belgrade, occupied Yugoslavia, from 15 September before completing his training at "Nachtjagdschule" 1, near Munich on 1 November 1941. During his advanced training, on 1 May and 1 August respectively, he was promoted to the rank of "Fähnrich" and "Oberfähnrich".