Make own setup file?

I'm trying to edit a setup file so that at the end of the setup, it also moves specific files to a certain directory. I'm fairly certain I can't just edit the already made setup file of the program, so I'm guessing I need to do a setup file that includes the original setup and then move the files on the next step?
I know there's a ton of free setup making software out there, I'm just not sure which can do what I'm looking for.
Any help would be greatly appreciated.

setup file for what? what type of installer? msi? nullsoft? installshield?
you could do it with a batch file.

It's an Installshield setup. I'm trying to attach something to it so when you finish installing the base software, it will automatically put an extension into a certain folder afterwards.
Then I need to rebuild the ISO and have the new setup as the autorun setup file.

Edit: I believe what I need can be done with Nullsoft(NSIS). Can anyone confirm? I would test it myself, but I'm stuck at work for awhile.
Edit 2: Ok, after some more looking, IExpress might work for me too. I don't need an installer to move the extension into the folder. I could just use some self-extracting file attached to the original setup.
I came across MSI Chaining, but it seems that's just for MSI setup files.

Perhaps it would be easier to help me if I gave an example. Let's say I'm running a small site that develops extensions for FireFox and I want to bundle my extensions with the normal FireFox installation. I want it to go through the normal FireFox setup and after it's complete, move my extension or theme or whatever into the appropriate folder automatically.

Autover can monitor two folders and automatically copy anything that changes in the first to the second. It even has network settings though I've never used it for anything but local files. All you should need do is create the first backup in autover and then when it starts it can automatically do it's job. Synkron will do basically the exact same thing but only on start. Autover will actually monitor the folders.
Batchrun can handle multiple batch files with wait commands in between if necessary and can chain multiple commands in one batch with parameters for each exe. I had it working once with clamwin portable so I could scan a particular folder right from the context menu. I used 2 separate batchrun files, one for the folder and one for a file. Clamwin got updated and I could never get it to work again. Oh well.