You MxAdmin does not have an account object. It's a system.user object. Therefore this will not work. You also will not be able to create an account with the name MxAdmin because Account is a generalization of User. And you cannot have two users with the same username ;).
Why do you want to MxAdmin to be able to do this? Maybe its better to just create an account for this (and maybe also give the admin rights) and use MxAdmin only als application admin.