Well, there's historical reality and there's the game.
The game should be balanced, and having the option of delaying the entry of the USSR (or even the US) for an inordinate number of turns would drastically unbalance the game.
Then there's history.
A good book on the period is:
"Grand Delusion: Stalin and the German Invasion of Russia" by Gabriel Gorodetsky
Did the Russians make offensive plans against Germany? Yes, but that is not evidence that they intended to carry them out. Stalin wanted to avoid war (or delay it for as long as possible), and the book argues that he over played his hand and made an huge miscalculation (the "Grand Delusion" being Stalins).
The USSR would never have attacked in 41 due to its ongoing re-organisation, and even in 1942, it's hard to imagine that the USSR would wait until Germany was supreme in Europe, and the Wehrmacht was "unemployed" before launching an attack.
When Molotov was in Berlin in Nov 1940, Ribbentrop even offered Molotov the chance to join the Axis (in the middle of a British air-raid) and the proposal was that the USSR expanded southward, but Molotov consistently made demands about Finland (officially in the Soviet "sphere of influence" but was occupied by German troops (at Finnish invitation of course)), and about Bulgaria and Rumania, and after meeting with Molotov on the Nov 13, Hitler was exasperated with Soviet intransigence and after the meeting issued "Instruction No. 18" regarding invading the USSR.
So, historically, the USSR could have realistically joined the Axis (after the war, Stalin is meant to have said "with the germans we would have been invincible"), and perhaps WWII would have been the US-UK versus Germany, Japan, Italy and the USSR (perhaps the Yalta conference in a parallel universe would have had a different photo of the Big 4).
In the original HOI, the Soviets could join the Axis I recall. But for a game like this, I think it has to be as it is.