Amending Movement types - re. Reversal/ follow on movement types - Help

Hi

I have copied a Reversal movement type 982 from a SAP standard 602

Now I wish to add movement type 982 to the reversal of movement type 981

How do i do this please ??

Currently there are no entries at all for the reversal of 981

Edit - New entries is also Greyed out

Please help

Many thanks