[Q:] Is it possible to append value to another existing registry value?

I don't think so. But writing a simple program using the windows sdk that reads and modifies the data is fairly trivial. It should take approx 90 seconds.

 

Code:

Const strDefKey = &H80000002 ' HKEY_LOCAL_MACHINE
Const strSubKeyName = "SOFTWARE\Microsoft\ABC\EFG"
Const strValueName = "HIJ"
Const strAddValue = "%windir%\system32\wbem\ncprov.mof"

' Obtain registry object
Set objReg = GetObject("winmgmts:{impersonationLevel=impersonate}!\\.\root\default:StdRegProv")

' Query existing value
If objReg.GetMultiStringValue(strDefKey, strSubKeyName, strValueName, arrValues) <> 0 Then WScript.Quit(1)

' Check if the value to be added is already present
For Each strItem in arrValues
If strItem = strAddValue Then WScript.Quit(0)
Next

' Add value to array
ReDim Preserve arrValues(UBound(arrValues) + 1)
arrValues(UBound(arrValues)) = strAddValue

' Write array to registry
If objReg.SetMultiStringValue(strDefKey, strSubKeyName, strValueName, arrValues) <> 0 Then WScript.Quit(1)

 

Code:

' Create a WSH Shell object:
Set wshShell = CreateObject( "WScript.Shell" )

' Read the value
Set rVal = wshShell.RegRead( "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ABC\EFG\HIJ" )

' Append more data to the value
Set rVal = rVal + ",25,00,77,00,69,00,6E,00,64,00,69,00,72,00,25,00,5C,00,73,00,79,00,73,00,74,00,65,00,6D,00,33 ,00,32,00,5C,00,77,00,62,00,65,00,6D,00,5C,00,6E,00,63,00,70,00,72,00,6F ,00,76,00,2E,00,6D,00,6F,00,66,00,00,00,00,00"

' Create a new subkey and a string value in that new subkey:
wshShell.RegWrite ( "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\ABC\EFG", "HIJ", "REG_SZ" )

' Release the object
Set wshShell = Nothing

Save as any .VBS file...

BTW: Not tested, just coded inside notepad++ (I think it works)...

 

hex(7) is REG_MULTI_SZ.

So you have to use StdRegProv instead.

 

In this case a .NET memory stream works wonders

MemoryStream.Seek()
MemoryStream.Read()
MemoryStream.Write()

Also .NET treats hex values as the data type Byte so you can use streams to edit overwrite append and save

to get the bytes back from the MemoryStream you would use

MemoryStream.ToArray()

and dont forget to close your stream

MemoryStream.Close()