My previous answer was correct. AAM can not subscribe to Customer Attributes.
You are able to onboard data into AAM directly but the process is a bit different. You may want to engage with the AAM support team for a detailed explanation. The documentation around this can be found here.
I believe most Audience Manager customers upload offline CRM data directly within the Audience Manager interface rather than using Customer Attributes. I don't believe there is an option to subscribe to Customer Attributes within the Customer Attributes interface by design.
I'm going to see if I can get one of my Audience Manager colleges to give his input here as well. Stay tuned....