SQL Server 2000-2014 migration using 2008 Express as intermediary
You could use Express, but it does not have all features that the full versions do and cannot handle databases larger than 10GB each. Please note the 10 GB restriction applies to the total size of data files only (so, for example, log files do not count toward the limit).

If that does not fit your needs, you could use a trial version of SQL Server 2008 R2 (ISO Image, or VHD) which is good for 180 days. As Aaron has researched, the EULA for the trial does cover this activity. The EULA states,


  
  You may install and use any number of copies of the software on your devices. You may use the software only to demonstrate and internally evaluate it. 
  You may not use the software in a live operating environment unless Microsoft permits you to do so under another agreement.
  


Which does apparently include the use of an evaluation copy as a stepping stone.

Either way, you should  use SQL Server 2008 R2 and not SQL Server 2008.