You can import custom fields. The ability to do this has always existed, since custom fields were created.

In your case, since the items already exist, the way to do this is to export your items list to an IIF file, which is a tab-delimited file, and then edit it in Excel, add the custom field values to the appropriate field (one of the CUSTFLD fields), then save the file (again, as a tab-delimited file) and then import it back into QuickBooks.

Note that it's best to remove all the item rows that you're not updating, since when you import an item on top of an existing item, matched by NAME, the item is essentially replaced with the data in the IIF file.

The items will look something like this in Excel. You'll see the defined custom fields at the top, and need to map them to the right CUSTFLD.  For example, the field "Color" is INDEX 0, meaning the first field and it maps to CUSTFLD1: