Filter Saved Report with API

I have a saved report that I need to manipulate externally.
However I need to filter by month, and this month is not consistent (Not always this month, or last month).

How do I either:
1)Add a filter parameter to an established query
2)Find the query string associated with a saved report
3) Utilize the "Ask the user" option in filters via API

Edit: Report is a summary report, or I could build the query string.