Jiabao,
Upon bom insert, there is a property called Table Template.
If you browse out for a different template, Solidworks (v?)
supplies a bom-weight and bom-materials among others.
If you only need to add a column to you bill, right click on the top of a column and add. The entered text can be saved, but not updated.
Try the Table Template first and proceed.
I am sure there is alot more help here than I offer.
I have used the bom-materials for weldment assem's.
Happy boming
dsgnr1
Experience is a dear teacher, and only fools will learn from no other.
Ben Franklin