If you are referring to 1939-40, Stalin did not seek satellite states so much as he sought additional territory that put more distance between Moscow and its western border. However it's difficult to uncouple this goal from the additional desire merely to control more territory that was lost to Russia during the early revolutionary period.

The Baltic states, Poland and Finland secured their independence in this period, along with the small region of Bessarabia transferring to Romanian control. An additional war with Poland led to more territory being transferred from the USSR to the Polish state. Between 1939 and 1941, the USSR marched into eastern Poland and annexed that territory, annexed the Baltic states, demanded the return of Bessarabia, and fought a war against Finland to modify their borders and secure basing rights in a key port. Finland's performance in the winter war probably saved them from becoming an outright puppet state - the "Finnish Democratic Republic" was a government established in December 1939 by the USSR but never installed in Finland proper. Whether this would have remained a puppet state or become it's own SSR is unknown.

In any case, I would not say that defensive preparations were the "main" reason Stalin took over these territories. It was one reason, but at the time of Stalin signing treaties with Germany and beginning the aggressive moves, there wasn't a strong sense of urgency regarding an imminent attack, nor even a guarantee that the Germans would win the conflict and defeat the Franco-British alliance.