IPv6 is definitely not built into Solaris 7 (as you have no doubt realised), and given that Solaris 7 was released in 1999 (i think), it's doubtful that there would be any support for IPv6, as it hadn't really taken off.
However... Solaris 9 is available for free download if upgrading is an option:
http://wwws.sun.com/software/solaris/binaries/get.html