You can add this inside the ForEach loop for the databases collection but you do need to have it formatted properly in Excel as each database would have at least two files
$fileGroups = $db.FileGroups
ForEach ($fg in $fileGroups)
{
$fg.Files | Select $db.Name, Name, FileName, size, UsedSpace
}
$db.LogFiles | Select $db.Name, Name, FileName, Size, UsedSpace