How to Mass Load Tag#'s in Asset Management?

We have a process to receive assets with Serial numbers but no Tag#'s. Tag#'s are assigned later after receipt. So we process receipts and Vouchers into AM - adding assets with serial #'s but no tag #'s. Later Tag#'s are assigned and we need to go back to the asset and add the Tag#.

The volume is huge. We order 5,000 chromebooks that need to be tagged and later added to the asset. How can we mass load these changes? I understand the CI is only for adds. I have a spreadsheet with all the Serial numbers and tag #'s. How can I load this data? Do I have to create a custom process?