Answer
No — custom fields can't be added to virtual-based entities in either IFS Cloud or on Apps10 Aurena. This is a hard architectural limitation, not a missing configuration option: the custom field mechanism relies on the entity actually being backed by a database table (or a proper base-table-backed view), and a virtual entity by definition doesn't have that. If you need custom-field-style data tied to the same business context, you'd generally need to find an alternate, table-backed entity related to the same data and add the custom field there instead, or pursue a genuine code-level customization.