Short answer: it is not possible.
Longer answer: You can do it but you will have to hack a good amount of the Q2A core and update any plugin that is using that. For example, see my profile url: http://www.question2answer.org/qa/user/pupi1985
. If this ended in /qa/user/123 then you'd have to modify this particular page so that 123 is interpreted as a userid rather than a handle. After doing so, you'd have to repeat the process for any other page that passes a handle in the url. Then you will have to modify each plugin so that it does the same thing.
And then, after modifying the core and plugins so much, forget about upgrading them by just replacing the files :)