altering WHERE clause when calling grid?

According to the documention you still only have one search field.
I have a huge table which is constantly growing, and my users want to narrow the fields down (based on several possible fields) and get
a) some summary
b) the naked data.

I have already a page doing this in pure php, but want to include it into joomla, displaying the naked data with TableJX.

So, can I somhow update the WHERE clause in {grid ...}, perhaps using Sourcerer?