This is similar to adding any new fields on OAF page and will be achieved using OAF personalization.
Go to OAF page >> Personalize Page >> Create Item >>
Item Style = Flex
Id – DFF (Any unique Value)
Appl Short Name – ICX ( Oracle iProcurement)
Name – TESTDFF (DFF Name) Click Here to create custom DFF
View Instance – OneTimeLocationDisplayVO (Associated VO of the page)
This will show the DFF available on the screen. Re order as per requirement.
Associated VO Details ( About This Page)
Note – Add Flex item on page layout region.