Before bulk uploading the SKUs for linking, you can export the existing unlinked SKUS (However, this step is optional).
There are two ways of bulk uploading the Product mapping:
1: Upload at Product mapping
a. On the path Channel -> Product Mapping, Click on Import icon
b. Download the template.
c. Update the given template with the values as in the exported file (as discussed in the step above) and upload it.
2. Import it on the Tools menu
a. On the path Tools -> Import
b. Select 'Channel Item Type' in the menu.
c. You always select 'CREATE NEW AND UPDATE EXISTING' in the import options, for this gives you freedom of creating a new SKU and even updating the existing product mapping in one step only.