Do you want to copy some data from one database to another?
If so, then a backup is not the way to do it. All you can do with a backup is to restore it in its entirety, either to create a new database or to totally overwrite an existing one.
You can write sql statements to reference tables in different databases by specifying the database name in the from clause. I think the format is databasename.dbo.tablename, but you had best check in BOL.