Hi Vijay,
As Rajendhar said, you should use business transaction '0000' (unspecified posting) with a Z variant.
You won't be able to add a new item category (your case is 07000-Fixed Assets) to an existing standard business transaction (your case 1000-Payments). In these cases (new item category needed) you should use BT '0000' where you have all item categories available and in the Z variant you set the "Forbid/Req./Only Once" characteristics.
So your customizing for ZP document type should be:
Type Bus.T. Variant
ZP 0000 Z001
Please see note: 1085921 - Document split, specifically section 2.9 Business transaction and business transaction variant.
Best Regards!
Mgitur
PS: Also see note 1492963 - Changing assgmt of item categories to bus. transac. variant