This is already how Python works. When you define y as you show, you are creating a Python variable, y, that has the value of a SymPy expression. So if you call some function, passing y, you are actually passin the expression:
>>> y = x**3 + x**2 + 3*x +5
>>> def foo(x):
... print 'got',x
...
>>> foo(y)
got x**3 + x**2 + 3*x + 5