jQuery Grid Plugin - jqGrid

I just started using the jQuery Grid plugin and I am having great success populating my grid and posting back data to a server running a proprietary API.

The one problem I am having is that I when Cell Editing I am posting back to my server extra parameters. The API I am using requires a record id parameter (rid) and the name of the field prefixed with "_fnm_". So if the name of the field I am posting is "lastname" I need to send this set of parameters

&rid=1&_fnm_lastname=Ferrells

I accomplish this by using the method beforeSubmitCell(rowid,cellname,value, iRow,iCol)

The parameters that are actually being set are the id and field name as defined in colModel in addition to the parameters I add by calling beforeSubmitCell(...). The parameter string look like this:

&rid=1&_fnm_lastname=Ferrells&lastname=Ferrells&id=1

My server is simply ignoring "lastname" and "id" and correcting updating the server database based on the parameters "rid" and "_fnm_lastname".

How can I configure jQuery Grid to not set the default id and field name "lastname" and only send what I return from beforeSubmitCell()? The nmethod beforeSubmitCell() seems to be *appending* additional parameters to the URL and not *replacing* the default parameters. I hope this is clear as I have scoured the documentation and forum without finding an answer.