Something as an "antechamber" for something else

In Italian there is the expression "something as an antechamber for something else", meaning something can precede and somehow cause something else.

For example:

Data show prisons are far from being places to rehabilitate criminals into society. For many young offenders, in fact, jail only represents an antechamber for more serious crimes.

or

Steam, the famous video game seller, now allows its customers to try a new game for a limited amount of time, hoping the rent would work as an antechamber for a purchase.

Now, while an English reader would probably understand what I am saying, I couldn't find this idiom anywhere. Is there any other idiom or metaphor that would allow one to express this same idea?

precursor
A person or thing that comes before another of the same kind; a forerunner:
In so doing, Coram created London's first art gallery, a precursor of the Royal Academy.

^{Oxford Dictionaries}

I'm not saying I agree or disagree with the statement below. It's just an example...

Long-term studies on patterns of drug usage among young people show that very few young people get started on more serious drugs without having used marijuana first. Therefore, marijuana is a precursor drug. People who use marijuana are more likely to use cocaine than those who have never used marijuana.

For the video game seller example you could say

... hoping the rent would act as a first step toward a purchase

In the US, at least, by far the most common metaphor used for "an entry point to a life of crime" is "gateway".

We see this, for example, in "gateway drug":

This is a study of the occurrence and timing of young people’s first use of various types of illicit drug and their first experience of various types of offending, including truancy. Its aim is to investigate the gateway effect – the hypothesis that use of soft drugs leads to a higher future risk of hard drug use and crime

^{Stephen Pudney, 2002: "The road to ruin: Sequences of initiation into drug use and offending by young people in Britain".}