How do you append an existing text file from a batch file?

I have recently asked how to set a value from the registry to a variable in a batch file for a current project I'm playing with purely to further my coding/programming knowledge, and I've hit another snag.
The batch file is basically going to be executed via an AUTORUN.INF file on a CD and, when run, install a Playstation emulator (completely legal providing those who use it own a Playstation and only use it for games they own). I'll paste in my code so far below to help you understand where I'm up to and where I want to go next, followed by an extract of the INI file about to be detailed.

The emulator has an INI file in the root directory which contains, in particular, the location of several storage folders. This is handy as it allows users to have non-default locations to suit their needs. My batch file installs all necessary files and directories to a hidden folder on the system drive (as shown in the code) except the "cdimages" folder which, using a registry key value saved to a temporary variable, is installed in the users My Documents (WinXP) or equivalent folder.
Seeing as my batch file allows for users having their My Documents folder in a non-default location the batch file needs to dynamically write the absolute patch to the cdimages folder to the INI file. Which is where my problem lies. I've seen plenty of websites telling me how to dump info to a new text file or to read data from one, but I can't find anywhere detailing how to append information to a set point in an existing file.
Note: I will also be using the solution for the "screenshots" folder.
If this is not possible then fair enough, but I felt this site would probably be my best bet if a solution does exist.

Brief description of the code thus far:
Lines 3 through 8 set variables and copy a free utility onto the host machine to allow the CMD to create a shortcut file (which I haven't coded yet).
9 through 16 create the necessary folders on the host machine.
17 through 24 copy the program's files to the newly created folders.
Line 32 is the path in question.

To be added:
A section which defines the output of my %docs% variable to the cdimages location of the INI.
The creation of the shortcut file using the CMD application, shortcut.exe.

I think I've explained the situation pretty well, but I wouldn't be surprised if I've missed something or forgotten to explain things.

1:
2:
3:
4:
5:
6:
7:
8:
9:
10:
11:
12:
13:
14:
15:
16:
17:
18:
19:
20:
21:
22:
23:
24:
25:
26:
27:
28:
29:
30:
31:
32:
33:
34:
35:
36:

REM this is the batch script up until the pause command.
@echo off
set dir=%cd%
set rootkey=HKEY_CURRENT_USER
set subkey=Software\Microsoft\Windows\CurrentVersion\Explorer\Shell Folders
set valuename=personal
for /f "tokens=1-2*" %%a in ('reg query "%rootkey%\%subkey%" /v %valuename% ^| find "REG_SZ"') do set docs=%%c
copy "%dir%\utils\shortcut.exe" "%windir%\system32"
mkdir "C:\PSx"
attrib +h "C:\PSx"
mkdir "C:\PSx\cards"
mkdir "C:\PSx\bios"
mkdir "C:\PSx\saves"
mkdir "C:\PSz\utils"
mkdir "%docs%\PSx\cdimages"
mkdir "%docs%\PSx\screenshots"
copy "%dir%\cards\*.bin" "C:\PSx\cards"
copy "%dir%\bios\*.bin" "C:\PSx\bios"
copy "%dir%\psxfin.exe" "C:\PSx"
copy "%dir%\psx.ini" "C:\PSx"
copy "%dir%\readme.txt" "C:\PSx"
copy "%dir%\unicows.dll" "C:\PSx"
copy "%dir%\unicows_license.txt" "C:\PSx"
copy "%dir%\utils\cdztool.exe" "C:\PSx\utils"
echo  
echo Installation complete.
pause
REM from here onwards is the extract of the INI, including the piece in question.
[Paths]
SaveStatePath="C:\PSx\saves"
MemoryCardPath="C:\PSx\cards"
CDImagePath=
ScreenShotsPath=
[BIOS]
PS1="C:\PSx\bios\SCPH1001.BIN"
PS2="C:\PSx\bios\scph39001.bin"