This is not possible in 6.0.x.
The field DialogUser.LastLogin is used only for the dynamic users. These users will be created automatically for role-based authentications. To delete the not-used users automatically the LastLogin can be used.
There are no events or similar that could be used for a custom defined function in 6.0.x. However, from 6.1.1, a feature is available to allow this to be tracked:
Activate the configuration parameter "Common\Journal\LoginAudit" and the system writes an information in the system journal (table DialogJournal).
A similar entry like this can be tracked in the DialogJournal:
Login succeeded for module Active Directory user account, properties: , identity: username, client machine: MACHINENAME
The system writes an information in DialogJournal after a successful login. One possible solution is to implement a database trigger for INSERTs in DialogJournal. This trigger can be used to write some additional information or fire another event.