If i where you i would go back to my functional consultant and suggest you add cashierID as an dimension in the system and fill in that dimension value in the G/L journal. That would save you and your customer for lots of customization and further system maintenance when moving to new versions. Easier for you and cheaper for your customer.
But in case your consultant does not agree with me.
You need to add the field to the G/L journal line and to all the ledger entry tables where you want to show your cashier id.
And then find an event trigger in Codeunit 12 where the G/L are posted that you can use to add your cashier ID to the different ledger entries.
And as i said - using a dimension for your cashierID instead would give you all of this for without customization.
