create custom table with standard table fields

Hi experts,

I need to create custom table for a new process in MM module by referring MSEG and MKPF table fields.

1 way : To add fields that we want to custom table from standard table.

2 way : Store only document no and year in custom table and when we need fetch data from standard table.

So I want to know what is the best way to do it..

Thank.

Pavi