Job ledger entries from sales invoice

Hi all,

I want to create job ledger entries (usage) when raising a sales invoice. In the Nav5 version, it's only possible to create a sales invoice from a planning or task line on the job. When creating a 'normal' sales invoice, Navision does not automatically generate job ledger entries, as there is no function which updates the job journal lines. When booking purchase lines and a job number is filled, Navision creates job entries and the job Usage total Cost (LCY) field in the Job Task Line table is updated. I want to create the same situation for sales invoices. Is there any possibility to create job ledger entries and update the calculated field Usage Total Price (LCY) in the Job Task table without creating too much code in the database?
To be short: how to update the Usage Total Price (LCY) in the Job Task Line table when creating a sales invoice, without using the 'create sales invoice from job' function?

Thanks,

Tom