The short answer is that WP Biographia uses the WordPress user system and API to gain access to a blog's users and to enhance their profiles with the extensions that WP Biographia adds.
There's no reason why WP Biographia couldn't be modified to work with WP Symposium but from looking at this plugin's code, WP Biographia can't work out of the box with WP Symposium.
There's several reasons for this. Firstly whilst WP Symposium still uses the standard WordPress user accounts, via the
wp_users table in the back-end database, it layers its own class functionality on top of
wp_users and uses the standard WordPress API only for the base user profile information.
If you wanted WP Biographia to support WP Symposium you'd need to modify WP Biographia to use WP Symposium's analog of the following API calls and WordPress classes ...
... there's probably a few more to add to that list, but this is off of the top of my head.
Additionally, as WP Biographia uses the standard WordPress avatar system (it looks like WP Symposium provides its own, non plugged version) you'd need to either provide WP Symposium with a plugged version of
get_avatar() or modify WP Biographia to use WP Symposium's avatar system.
Like I said, none of this is technically impossible, but it would need a reasonable amount of code changes in both plugins to allow them to function well together, and even more code changes to allow them to optionally interwork together.
If you want to do this, then feel free to go ahead. WP Biographia's source is freely available for forking on GitHub at https://github.com/vicchi/wp-biographia.
I'd be happy to advise and give limited help, but I hope you can appreciate that I just don't have enough spare time to be able to make these (substantial) code changes in a future release of WP Biographia, at least not for the foreseeable future. Of course, if loads of people clamour in with "me too" then I'll happily look at this again!