Is there a way to manually register a user with a py-transport server-side?
Go through once for each transport
and register yourself.  Capture the
XMPP packets. 
Dump the transport
registration data from your current
system into a csv file, xml file, or
something else you can know the
structure.
Write a script
using jabberpy, xmpppy, pyxmpp, or
whatever, and emulate each of your
users registering with the
transports.

One issue is you may have to be connected to the Internet for the transports to come online.  Then you're going live with someone else's account.  If you can't get their current password data for your jabber server, set it all to a default and then migrate it back after your transport registration.
I'm trying to write some scripts to migrate my users to ejabberd, but 
the only way that's been suggested for me to register a user with a 
transport is to have them use their client and discover the service. 
Certainly there is a way, right?