No you can not take a DB2 backup and restore it to a SQL 2005 database. You will only be able to restore it to another DB2 database. In order to transfer your data from a DB2 system to a SQL Server (or any other database) is a manual process.
First you will need to create all the tables. You can usually do this by scripting out the tables using the DB2 tools then making some modifications to those scripts and running them against the SQL Server database. You can then use the SQL Server Integration Services to migrate the data from the DB2 database into the SQL Server database.
You will only be able to restore it to another DB2 database.
Actually, you can only restore it to another AS/400 that is at a compatible release level. The AS/400 backup cannot be restored to DB2 on any other platform. If a migration is intended, then the system backup facility is not appropriate.