In this case you would need to create (manually) all customers as G/L accounts as well and also create one customer posting group per customer and assign the right G/L account to the right customer posting group and the right customer.
This could be supported by some code which would be needed to automate this behavior.
From a accountants view: You are requesting for a solution which will nulify the subledger concept of NAV. The requirement seems to be originated from the leagcy system which client might be using.
G/L & sub-ledger have their own importance. You cannot pull all the info of the sub-ledgers in the main G/L as it will make it bulky as well as difficult to analyze. The trial balance will run in hundreds of pages (assuming just 1000 customer/vendor each). Implementation of ERP is more than data punching and statutory report preparation. The management users should be able to analyze the summary and should have the option of drilling down, if required. The client should adapt to the new system and its methods.