Sorry for the confusion, but we are not sure what exactly you mean. There is only one property provided by AD that is used to specify user managers. The name of the property is Manager. As an option, you might use other DN syntax properties (e.g. Secretary, See Also), but they do not have backlinks like the Direct Reports property for the Manager one.
On the other hand, if you need Adaxes to always send account/password expiration notifications to the same email address, you can predefine it in the Send email notification action settings. In this case, you will not need to specify anything for user accounts at all.