By all means try that, and there are no prerequisites for FP5 (XR_C005).
The way I did it successfully for FP6 (XR_C006) is as follows:
1. Grab a copy of the latest FixTool and extract it somewhere, e.g. C:\Fixes\FixTool
2. Copy the FP6 files into a temporary folder, e.g. C:\Fixes\FixPak6
3. Set two environment variables as follows:
SET CSFUTILPATH=C:\FIXES\FIXTOOL
SET CSFCDROMDIR=C:\FIXES\FIXPAK6
4. Now "CD" into C:\Fixes\FixTool and run SERVICE without any parameters
That should work for you, and once you've rebooted you should then "commit" the FixPak to cleanup temporary files and for neatness (actually perhaps someone else could chime-in here to better explain what "commiting" a fixpak actually does! I think it simply prevents rollback to pre-FixPak).
To commit it, repeat steps (3) and (4) above and select Change product list... > Uncommitted products > Commit
If you don't have a copy of the latest FixTool just shout and I can attach it - it's freely available on IBM's FTP server and many other locations.
Hope that helps!