AAM offers server to server integrations with both Krux and Lotame. Please review the following link  for a list of available integrations with AAM. You will need to filter by App Type > Destination.
Yes, that is possible with AAM. You can send data as well as onboard data from other DMPs to AAM.
You need to contact Adobe Customer Experience team to get the server to server integration set up, and enable partner ID syncs with the other DMP. Please check with the customer experience team, which DMPs are supported, I believe Krux and Lotame are supported.