I think you are out of luck unfortunately. UFS filesystems generally can't be grown. I believe your only choice would be to make a full backup, reinstall solaris laying out the filesystems how you want, then restore from your backup.

If you are using Solaris disksuite you might be able to get around that limitation, but I don't know how. I know with Veritas Volume Manager you can increase a volume and filesystem on the fly. But even then, if it is your encapsulated root volume it is an awfully painful process.