I'm not really sure why you need that sessionToken. The sessionToken is browser specific and is associated with a Java Bean in bbb-web (tomcat). The token can't be transferred and used elsewhere. The token also can't be generated by your front-end because bbb-web needs to create it.
If you want your users to join with different names all you need to do is generate a join request for that user. A good tool for exploring the different API properties and URLs is Mconf's API Mate
http://mconf.github.io/api-mate/. Every time you change the "fullName" property a new join URL is created.