ABC,
Unfortunately, it is not directly possible in any SQL Server versions,
including SQL Server 2005 as you must have a unique, single non-nullable
column to FT-enabled a table... That said, the workaround is easy. Alter the
table and add a int column with the identity property, for example:

use Northwind
go
exec sp_help EmployeeTerritories -- PK is PK_EmployeeTerritories
(EmployeeID, TerritoryID)
go
-- Alter Table: add unique, single non-nullable column
ALTER TABLE EmployeeTerritories ADD ET_Ident int identity (1, 1) NOT NULL
go
CREATE UNIQUE INDEX ET_Ident_IDX on EmployeeTerritories(ET_Ident)
go
exec sp_help EmployeeTerritories

Hope that helps!
John