Thanks for sharing your query here on this thread, hsanchez. Let me provide information to help you import data and manage custom fields in your account.
With QuickBooks Online, you can only import specific fields and data into the program. For now, the ability to import custom fields from Excel during the mapping process is unavailable.
You can always reply to this forum if there's anything else you need further assistance with when managing custom fields in your account. Feel free to let me know, and I'll be glad to provide additional help. Stay safe and have a great day ahead!