Very good question. In order to load an user you need to know the partition he belongs to. So, the quick answer is no.
Usually, people are extracting the partition from the user name. Something like "luis@partitionA". Where "partitionA" refers (directly or indirectly) to the name of a partition where the user belongs.
Also, please remember that an user with the same name can exists in different partitions. It can be tricky the way you find the partition an user belongs. This can lead to a another requirement where if an user was found in multiple partitions you should present him a specific page for selection.