I wonder why you can't use stepping stone; after all, the Monkees did:
When I first met you girl you didn't have no shoes
Now you're walking 'round like you're front page news
You've been awful careful 'bout the friends you choose
But you won't find my name in your book of Who's Who.
I I I I I'm not your stepping stone
A stepping stone can be used for progress in a neutral way (as in, the minor leagues were a stepping stone for the star third baseman), or it can be used with a spirit of maliciousness or selfishness, as in your example. When the person using the stepping stone does so in a self-serving way, I think you can use context to convey that negativity, without changing the expression stepping stone.
For example, you might be able to use a word like "just" to help express this sentiment. Compare:
His time in college was a stepping stone to his pro career.
His time in college was just a stepping stone to his pro career.
In the latter sentence, the word just trivializes the college experience, while in the former sentence, college seems to be more valued as a developmental step.