Fields auto copying and graded out in purchasing documents

Dear All Gurus,

as we all know when we create purchasing documents like PO ; when we enter currency as USD then this field is get copied to every below line and total column is get grayed out so we can not change.

this functionality i needed for plant field so that PO items will only have one plant what user have entered in first line.

Pl suggest how i can do this

Regards,

Vimlesh