Uploading users without password, moodle will build the password and sent it to the user.

It seemed, that the field {$a->firstname} also send the lastname. And the field {$a->lastname} will be as field in the e-mail.

In the screeshot you can see:
"Dag" is the firstname and "Klimas(Test)" is the lastname.
{$a->lastname} is the field added in the language settings for "newusernewpasswordtext"