In the Building Large IPv6 Service Provider Networks webinar I described how Cisco IOS uses two RADIUS requests to authenticate an IPv6 user (request#1) and get the delegated prefix (request#2). The second request is sent with a modified username (-dhcpv6 is appended to the original username) and an empty password (the fact that is conveniently glossed over in all Cisco documentation I found).
FreeRADIUS server is smart enough to bark at an empty password, to force the RADIUS server to accept a username with no password you have to use Auth-Type := Accept:
Site-A-dhcpv6   Auth-Type := Accept        cisco-avpair = "ipv6:prefix#1=fec0:1:2400:1100::/56"

Hi, Are there any chances to provide DHCPv6 server in FreeRADIUS?

Dhcpy6d delivers IPv6 addresses and prefixes for DHCPv6 clients, which can be identified by DUID, hostname or MAC address as in the good old IPv4 days. Addresses may be generated randomly, by range, by arbitrary ID or MAC address. Clients can get more than one address, leases and client configuration can be stored in databases and DNS can be updated dynamically. Range-based prefixes be delegated as well as fixed prefixes per client. Runs well on Linux and *BSD.