Sure you can do this but you probably shouldn't <s>. Especially in the scenario you describe because I assume there's actually a little bit of code involved in these custom controls.
Given that in VS you can create folders and break out classes into any structure you might need it seems like it makes a lot more sense to have separate files that you can easily identify vs. having to dig through 50 classes in a single file.

For the longest time, I had my Winform subclassed controls in one class file, and I thought that breaking them out was not a good idea. Then I worked on a project where someone had previously created one .cs file per control...and I wound up having to make some tweaks...and realized it wasn't the big issue that I thought it might be.