afraid you cannot. can you become root?
First disconnect from any network, login as root (if cannot and don't know how, hit Ctrl-Alt-F2 (e.g.) and type in
sudo passwd root
to create a root password.
Then login as root and use the commands
useradd -m xxxxxx
to add a user and his home folder, then
You could then delete the olduser and his home folder if you want by
userdel -r olduser
I've done it, too. C'est la vie!