RobertaME

Yes, it is entirely possible to edit other people's CSOs.

I did this with nothing more than Notepad++ and its Hex Editor plugin to give the ride a cost (nothing should be free) and change its name. (still looking into OVL code to find more things that can be changed without unpacking them and trying to repackage them entirely... which is neigh unto impossible in many cases)
The REAL question is "Is it allowed/ethical to upload fixed versions of a CSO for other people to use?" I edited my own downloads of these files for my own use, so it's no big deal... but whether or not I should be able to (or be ethically justified in) uploading those for others' use is a very contentious subject. I look at it as an homage, giving credit to the original creator and adding to what they've done... the creator might think differently, though. ::shrug::

Can confirm, there are no Popcorn vendors in RCT3. Since I have Custom Popcorn stalls installed, what I do is have them sell Fries renamed as Popcorn.

Since what the peeps are carrying is just a bag with something yellow inside, it works and has the desired effect of making peeps thirsty while satisfying hunger slightly.
YMMV.
 

The CTR Creator 1.00 by Markus Lomberg has under the Tools menu a "Recover lost MOD/XML files from Model.ovl" button that will usually extract texture files as BMPs and 3d models as XML files that can be changed and then made into new OVLs using the RCT3 Importer v18b2 by Belgabor. It's one of the ways I've found to change scenery objects that people have made over the years... extract the XML and textures using CTR Creator, make any alterations I need, then create new OVLs using the Importer. (eventually I'll get around to writing up the entire process in my "Editing OVL Files" tutorial)
The main issue is that the CTR Creator can't read ALL .ovl files... so there are times this process doesn't work. However, after checking it myself, I find that it CAN unpack the model data from the walls you're looking to use. (at least, I think so... assuming that they're the files found under Style\Vanilla\WallSets\FlexiWallSet) I'm unsure though where the texture data is since these files don't seem to have texture data in them. (actually, I just found them under Style\Vanilla\WallSets\FlexiWallSet\FlexiWallSet01_Textures.common.ovl under the extracted files flexibasewalls.bmp and flexibasewalls-Alpha.bmp)
flexibasewalls.bmp
flexibasewalls-Alpha.bmp
Hope this helps! :^)