Is there a way to upload values to customer (member) custom fields?

Hello, @treasurer_leanderband.

Currently, there isn't a way to upload the customer's custom fields in QuickBooks Online. As a workaround, I'd suggest utilizing the Customer Type feature. I'm happy to provide detailed steps and screenshots to accomplish this.

Let's create a type first by going to Sales and choose the Customer tab. Then hit Customer Types and enter a name. 

After that, you can now assign the type to your three members by following the steps below. 

1. Select Sales, then choose Customers.
2. Check off the three customers and hit the Batch actions drop-down. 
3. Choose Select customer type.
4. Pick a type from the drop-down, then tap Apply.
5. Click the small gear icon on the left side. 
6. Put a checkmark on the Customer Type box. 

For the detailed steps, check out this article: Set up and assign customer types in QuickBooks Online.

You can always open a sales report to review the data by going to the Report tab. These includes the following: 

• Sales by Customer Type Detail report
• Sales by Customer Detail report grouped by customer type
• Customer Contact List report customized to add a customer type column

Also, you have an option to hit the Save customization tab to memorized the report. For more information, check out this page: Memorize reports in QuickBooks Online.

Additionally, if you're using QuickBooks Online Advance, you may refer to this article for more knowledge bout custom fields: Create and edit custom fields in QuickBooks Online Advanced.

Please browse this article about managing customer's transactions and information for your future tasks: Sales and customers

Let me know if you have other concerns. I'm always here for you. Keep safe!

@treasurer_leanderband 

Explore an importer tool as a workaround.

https:// transactionpro.grsm.io/qbo