How to add a foreign currency to an existing Vendor?

It's nice to have you in the Community, @Sc0. I'd be delighted to assist you in adding a foreign currency to the existing vendors in QuickBooks Online. 

In the meantime, you can’t change the currency of a vendor once you’ve added transactions with them. As a workaround, I’d suggest making the existing vendor profile inactive and adding the vendor again with their foreign currency. Here’s how: 

1. Go to Get paid & pay or Expenses, then select Vendors.
2. Select Vendors, then New vendor.
3. Enter the vendor's name and other info.
4. From the I pay this vendor with ▼ dropdown menu, select the currency you use to pay this vendor.
5. Click Save when you’re done.
   
   

You can read through this article for more information about adding vendors with foreign currency: Add a vendor with a foreign currency in QuickBooks Online.

Additionally, I'll share with you these resources that may come in handy: 

• Assign a foreign currency to a customer in QuickBooks Online
• About Multicurrency

I’m always ready to assist you if you have any other questions or concerns about managing your vendors in QBO. Tag me in your reply and I’ll sprint back into action. Have a good one and keep safe.