As suggested, I selected all unread messages and clicked export. I chose "import compatible export" and I selected just "notifications/read". After exporting I get a file like that:
"id","notification_ids/is_read"
"__export__.mail_message_8003","False"
"__export__.mail_message_7992","False"
"__export__.mail_message_7984","False"
I changed "False" to "True" and tried to import. Odoo detects the columns correctly, but I am getting this validation error:
null value in column "partner_id" violates not-null constraint DETAIL: Failing row contains (14847, t, f, null, 8003). in Zeile 2
null value in column "partner_id" violates not-null constraint DETAIL: Failing row contains (14848, t, f, null, 7992). in Zeile 3
null value in column "partner_id" violates not-null constraint DETAIL: Failing row contains (14849, t, f, null, 7984). in Zeile 4
+1 For the question. Im looking for any solution too.