dir c:\temp\*.bak | ForEach-Object { & "C:\Program Files\7-Zip\7z.exe" a -sdel ($_.DirectoryName+"\"+$_.Name+".7z") ($_.DirectoryName+"\"+$_.Name)}

# To do this to all .bak files in all subfolder, just add -Recurse to command:
# dir -Recurse c:\temp\*.bak | ForEach-Object { & "C:\Program Files\7-Zip\7z.exe" a -sdel ($_.DirectoryName+"\"+$_.Name+".7z") ($_.DirectoryName+"\"+$_.Name)}