SQL Server 2012 (v11.0) doesn't support direct upgrade from SQL Server 2000 (v8.0).
You'll have to use a "in-between" step restoring your SQL Server 2000 database to a SQL Server 2005 or 2008 instance, and then do a backup from there, in order to get your .bak
file into the latest database server.
There's no trick or workaround for this - that's just the way it is, and you'll have to deal with it.
There are tons of blog posts on this topic, e.g.