Insert Trigger Question

Is there a way to update the value of a column in an insert trigger? For example, a row gets inserted (via a non-user initiated event) with a null value for one of the columns. I want a trigger to fire which calculates the value of this column (and assigns it) based upon a complex formula of other values already in the table (preferably from a return value of a stored proc). But, from the manuals (Online 7.2), it seems that this kind of calculation cannot be performed. Could this really be true? Is there another way to accomplish this task with Informix? Thanks for any help, Chad