How can I mass add subsidiaries to vendor records?

I have a load of vendors that I've added using CSV import.

Currently they're paired to the primary subsidiary MPCN and use a currency of CNY. But I want to add to all of them a second subsidiary and currency.

I see I can likely add additional currencies using the CSV but for subsidiary I only see the option to add a Primary subsidiary and no other. I also checked mass update feature but it doesn't show fields from any of the sub lists where the subsidiary field is in. So is there any way to achieve this?

Thanks!