It is definitely possible to add a proc entry in a kernel module but you might be misunderstanding how file handling works in the kernel.
When you create a file in proc, you're not actually 'creating' a file like you would in userspace. You're registering a few callbacks that are called when userspace programs request your file.
That means, when userspace programs request a read, your file read callback has to provide data. When a userspace program requests a write, your file write callback has to handle it.
If you want to use it like a conventional file to store information, you have to allocate the required space and have your read callback copy data from there.