passing an expression as a parameter to an Expression transformation (or Filter or AGG or whatever) is quite possible. This works as follows:
When you define a mapping parameter, define it with type String and (that's the most important detail) check the check box IsExprVar (short for "is expression variable").
Now when you pass a string for this parameter via a parameter file (or a pre-session assignment from a workflow variable of type String, that works just as well), this string will be treated as an expression term instead of a fixed constant value.
The only catch is that the whole expression term must be indicated as one single text line in the parameter file; you know, you cannot split a parameter value over several lines, the whole expression term must fit in one text line.
For example, you can set up a parameter $$CHECK_COND of type String(1000) and - assuming that there are two input or I/O ports CHECK_TYPE and CHECK_VALUE in the same EXP where you want to use $$CHECK_COND - pass the following line in the parameter file:
$$CHECK_COND=IN( CHECK_TYPE, 'A', 'C', 'M') AND (CHECK_VALUE = 2000 OR (CHECK_VALUE > 500 AND CHECK_VALUE < 1000))
Of course the parameterised condition must only use ports which are available in the transformation where you want to use $$CHECK_COND.