Answer
Yes — create the first two fields as persistent/numeric custom fields for manual entry, then create the third field as a Read-Only Expression-type custom field whose expression simply adds the first two together. However, wrap each input field in NVL(..., 0) within that expression, for example NVL(cf$_field1, 0) + NVL(cf$_field2, 0), rather than adding the raw field references directly — plain SQL addition against a NULL value returns NULL for the whole expression (1 + NULL = NULL), while the NVL-wrapped version correctly falls back to treating a not-yet-entered field as zero, giving the expected sum instead of a blank result whenever either input is still empty.