If you have the song files, but no .cab archive, you can simply copy the song files to the PowerWorship /Songs folder, using Windows Explorer. To access the /Songs directory, first check the path in the PowerWorship settings (Edit menu > Settings, Environment tab), and then copy and paste the song path into the Windows explorer address box.
Note that the default song folder location on Windows 7 (for PowerWorship 3.2) is C:/ProgramData/PowerWorship/pW3/Songs. On Windows XP, the folder is under C:/Documents and Settings.
Once you have copied the song files (*.txt), restart PowerWorship, if needed.
If you have old group files, note that you can copy them into the PowerWorship /Groups folder.