I've tried to figure how to add weight to BOM (that uppdates automatically from mass properties). Is there a way to do it or do I just have to add it manually?
Link the mass to a custom property (research custom properties in the help file).
Go to the custom properties manager ("File-->Properties" and click on the "Custom" tab). Here is where you make custom properties. You can make a property titled "part_mass" and link its value to any of the mass properties by clicking the "Mass Properties" button.
What TheTick said is what I have been doing too. He is pretty smart sometimes
If you put a Custom Property in each part like he stated, you can add the column to your Excel BOM pretty easy. From inside your BOM template, insert a column anywhere (left of the "$$END" one though) and using Excels name function (Insert | Name | Define), use the custom property name (part_mass) as the cell name. If you add a column L, then make cell "L1" have a name of part_mass. You can write WEIGHT in L1 as it's "value" also, so it will show up in your BOM.
One caveat regarding mass properties in custom properties:
The units of the mass properties are not governed by the units setting in the Document Properties. They are governed by the units set under "Options" in the Mass Properties tool (See "Tools->Mass Properties" or click the "blue balance" icon on the "Tools" toolbar).