How to change unknown master code to dummy code while import transaction data from bw using conversion file

hi expert

please let me know How to change unknown master code to dummy code while import transaction data from bw using conversion file

for example)

source data

C001 1,000

C002 1,000

C003 1,000

C004 1,000

C005 1,000

C006 1,000

C007 1,000

Entity Dimension

C001

C002

C003

C004

C005

CDMY

Changed(imported) data

C001 1,000

C002 1,000

C003 1,000

C004 1,000

C005 1,000

CDMY 2,000

I like to change not registered entity code to dummy entity code

Is there any way to change master code using Conversion File ?

OR NOT

Please tell me how to ..

thanks