You are on the right track.
asadmin create-file-user <username>
Should acutally prompt you for password and then update the keyfile under domains/domain1/config
With two users j2ee and wsit, the keyfile in my GF looks like this :
j2ee;{SSHA}dtvt9fI/M+W6sDCyRI7ddQ/tggA6g6C48neAgQ==;
wsit;{SSHA}JKCxqT6FUKgz0CX1AFzLIE5HHRj/PFZDskwpfw==;
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