You can inkoe the "date" command on Linux/Unix as command tasks, but you cannot use these commands in parameter files.
Meaning you can use these commands to produce a parameter file, but PowerCenter can't execute them from within a parameter file.
I think I would change the approach for this.
You have a reference date to which you want to perform the add_to_date to.
I would create to mapping parameters for just the value -15 and -1.
In every expression you can use this combined with the add_to_date function or even in the source qualifier as value for the sql in the source filter.
How to use this basically depends on your requirements.