You would simply have to set the taming item ID for the same item for all Pokemon; in this example, the taming item is 619 (Unripe Apple).
2002,BULBASAUR,Bulbasaur,619,9001,10013,531,80,20,50,100,250,20,2000,400,1,0,350,400,800,{ petloot 10; }
2003,IVYSAUR,Ivysaur,619,9001,10013,531,80,20,50,100,250,20,2000,400,1,0,350,400,800,{ petloot 10; }
2004,VENUSAUR,Venusaur,619,9001,10013,531,80,20,50,100,250,20,2000,400,1,0,350,400,800,{ petloot 10; }