You can't overwrite fields in the bom.
The whole idea is that the model is the master and holds the information required to populate the bom fields.
Any property and most variables in the mdel can be accessed in the bom.
Select the bom in your draft file, then RMB and properties.
Click on the 'columns' tab and select the properties you require. You can set widths and headings etc here also.
If they are not there you will need to add them to your parts.
Unfortunately you WILL require write access to the part files to do this - it's a basic requirement for any file that you want to edit, not just SolidEdge.
To add them, I would suggest creating a part with the properties you require and the values. If the values are common enter it, otherwise use a "-". Then use Property manager to open this file and the others you want to add the properties to. To start proprty manager in this way start SolidEdge but before you epen a file go to Tools>Property Manager. You will be presented with a dialog box where you can select the files you want to open in property manager. Just highlight the ones you want and the click open.
Set the columns in property manager to show these new properties and fill in the properties for the parts that require them. You can use copy/paste, and also find/replace.
bc