There is a standard SAP service (paid service) available for existing materials to be converted to Batch managed, but you can search this with "Batch Management Conversion Tool
(Report RVBCONVMAT2BM)".
You can convert your inventory of more than thousands of SKUs within just couple of hrs to batch managed and all your documents and master data will be updated and as compared to the effort required by doing it maually its the cheapest option available. It costs less than 10000USD I think.