Arturo,

Im not aware of any tools available to export local users database from vpn concentrator to ASA applience, however, a good approache in this type of migration scenario is to integrate IAS Radius and authenticate your users through active directory on your ASA when configuring RA, if you have special users that are not part of your AD domain accounts and that require local user account you could create a second RA tunnel for these type of users and have them authenticate through LOCAL users database in ASA. All this can be done transparently meaning you could have the VPN3000 and ASA firewall both running in parallel for RA until the implementation on ASA is complete.

Rgds

-Jorge