There is an import fuction in Lansweeper, but it's not designed for updating assets, only importing new ones. Unfortunately, in your case there is no other option than inserting the data into the database with a script.
I would assume you want to add the asset tag as an Asset custom field. These can be found in the tblassetcustom table. If you want to add the asset tag to an asset which matches the serial number it would look something like:
Update tblAssetCustom
set Custom1 = '#12345'
where serialnumber = '012801021749'