Document Currency in VF01- Modifiable

D Friends,

Our requirement is as follows:

1. In case of inter-co. STO to US/ Europe we need to change document currency in invoice (VF01). But since header document currency field (VBRK-WAERK) is non-modifiable so we every-time change the currency in Customer Master.
2. I do not want to change customer master every-time. So kindly let me know how to make header document currency field (VBRK-WAERK) modifiable in the VF01/ VF02.

Thanks.