need to insert data from oracle form to new fields of same table

Hello All,

1) I have 2 block A(master) and B(detail)
2) Detail table has says 6 fields: Field 1,Field 2, Field 3, Field 4, Field 5, Field 6
3) Right now Field1 and Field2 is getting populated from Block A(master) and Field3 is populating from Block B(detail)
4) Requirement is to copy
    Field 4 := Field 1
    Field 5 := Field 2
    Field 6 := Field 3
Also Field 3 can later be updated by users but once value got copied in Feild 6 that value should not be updated(basically it should get populated when record gets created.

Imp: Field 4, Field 5, Field 6 are not on form layout. I just need to populate table