linking customer invoice with internal order

Hi All,

Can anybody Please tell me What is the procedure to link the customer invoice with internal order?

Presently my client is not having SD module,

Present position is like below:

Once they receive the customer invoice/Agreements through mails they are updating manually in the system, in a different software.

but now they want to update that details in the internal order for analysis purpose.

Is it possible to enter that details directly in the SAP and link it to Internal orders

Could anybody help in this!!!

Thanks

kishore