When you import a product there is the option to click at the beggining of the column 

"create if does not exist"

I am trying to do an importation doing that and it does not work for me, I do't know why, maybe a bug? or probabbly I am doing something wrong, I am not an expert. But I think this is the way to do that, you can try to export different field of a product and see the field that you need to import to set the attributes.

I hope it helps, regards,

Amadeo

Unfortunately this is not simple to do.  For example, Odoo would not know whether a new variant was a colour or a size.

I found one third-party app:  https://apps.odoo.com/apps/modules/13.0/sh_import_product_var/ (but I haven't used it, so I can't offer any comment on it).