If you are, for example, running a rule then you can do anything, including reading data from an external location.
Or you could add a scheduled job in a similar way that user accounts get pulled from LDAP.
However you are exposing Alfresco to an external dependency so you would need to consider very carefuly what you are trying to achieve, and what happens if that dependency is not available or is compromised. Yes its possible, however the bigger question is whether you should.