Using Out-file -Appended switch, WMIC output in file has unwanted NUL codes in Notepad++

Not sure why you would want to merge csv output with some other kind of text, but just use native methods instead of wmic:

Get-WmiObject -Class Win32_LogicalDisk -Filter "DriveType=3" -ComputerName d2j7jsr1 |
	Select-Object -Property Name, FreeSpace, SystemName, FileSystem, Size, VolumeSerialnumber |
	ConvertTo-Csv -NoTypeInformation |
	Add-Content -Path $FNLog

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If all you want is a csv with the disk information, you can use Export-Csv directly:

Get-WmiObject -Class Win32_LogicalDisk -Filter "DriveType=3" -ComputerName d2j7jsr1 |
	Select-Object -Property Name, FreeSpace, SystemName, FileSystem, Size, VolumeSerialnumber |
	Export-Csv -NoTypeInformation -Path $FNLog

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>  Not sure why you would want to merge csv output with some other kind of text, but just use native methods instead of wmic:

I think I'm stuck in old ways and my ignorance of powershell haha.

As it is setup currently, ps creates the cmd file with multiple robocopy lines. At the end, I add the wmic line to get the disk space. Example:

robocopy "z:\cad\pjs\_MISCELLANEOUS-DETAILS" "g:\dir\Backup\BupOfRecentPjs\cad\pjs\_MISCELLANEOUS-DETAILS"   /xf Thumbs.db *.dwl* /e /np /nfl /ndl /r:0 /w:0 /dcopy:t /log+:"c:\local\CopyPjs10DaysOld_Rslt_Acad.txt"
robocopy "z:\cad\pjs\_UTILITY_NOTES" "g:\dir\Backup\BupOfRecentPjs\cad\pjs\_UTILITY_NOTES"   /xf Thumbs.db *.dwl* /e /np /nfl /ndl /r:0 /w:0 /dcopy:t /log+:"c:\local\CopyPjs10DaysOld_Rslt_Acad.txt"
wmic /node:d2j7jsr1 logicaldisk where drivetype=3 get name, freespace, systemname, filesystem, size, volumeserialnumber >>"c:\local\CopyPjs10DaysOld_Rslt_Acad.txt"

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How would I add your first example as a ps call from that cmd file to replace my current wmic call?

I'm not quite following what you're trying to do here, sorry.
Why and how exactly are you creating a cmd file from PowerShell? Just run the robocopy commands from PowerShell.