Adding one more selection field in the standard data source

Hi SAP Gurus,

I'd like to know how to add one field as a selection field for the data source.I used 0FI_GL_4 Datasource .It has 2 selection.Now I want to add one more field as a selection field .How to mark that field as selection?I tried by using RSA6 ,But I'm unable to change the previous selection.

Please let me know how to fix it?

I'll assign the points ...

Thanks & regards

Raja