With RoundCube 0.2 Beta only this works:
1) Create a user for the website in ISPConfig
2) Login into RoundCube with the Login name and password as defined in ISPConfig for the mailuser.
3) Go to the identities tab within RoundCube, there you see:
Loginname: as defined in ISPConfig
E-mail address: firstname.lastname@example.org
Displaying the realname is not possible indeed.
Personally, i think it is not a real problem, because everything works.
If the user doesn't like the the name which is displayed, it can be modified easily.