If your BOMs are in Excel, then Pivot Tables would be a good way to merge them. These are a native Excel tool, no VBA needed. You can create a master BOM Excel file, and set up a Pivot Table with the individual board-specific BOM files defined as its data sources. You can then format the table to group and sum the BOM lines however you want, and use the result to place your orders. Once it's setup, if you modify a board you should be able to have Altium generate a new BOM, and then simply refresh the Pivot Table to have it reflect those changes in your master BOM.