ConditionExpression queryCondition = new ConditionExpression("new_principal", ConditionOperator.Equal, principal.Id);
FilterExpression fexp = new FilterExpression()
{
FilterOperator=LogicalOperator.And,
Conditions = {queryCondition }
};
qe.Criteria.AddFilter(fexp);
context.InputParameters["Query"] = qe;
Like this u can add condtions. you can modify the query how ever you want.
This is possible in Pre-Operation.
In Post-Operation Plug-In you can't modify the query that to you can't get the query parameters as expected.