Some more info...
After going through some threads and forums, I found out some important stuffs. The file structure of iOS 5 is quite different than previous version of iOSs. The carrier bundles file in System/Library/Carrier Bundles/iPhone is the main directory where all the carrier bundles are centrally located. If the phone carrier doesn't match with the list of carriers pre-built in the iPhone, it uses files from unknown.bundle inside this directory. So we need to replace the .png files from this (unknown.bundle) directory using some SSH (ifile, ifunbox, etc.).
I also came to know that we must first patch the commcenter as it directs which bundle to use. So, I found out an updated CommCenterClassic Patch for iOS 5.0.1 from v.backspace.jp repo (you can add this in cydia and install it). Only then can we edit carrier.plist files.
I also want to register my carrier with the iphone so that I can automatically add APN, MMS APN, and other services to my sim. And, since my carrier (from Nepal) is not registered with apple there is one way to add the carrier bundle to the phone but the service is available to iOSs before 4.2.1 only. This one way is to create a carrier bundle file (.ipcc) and update it to iphone using iTunes update. I couldn't find anything for 5.0.1.
I am still all mixed up but I'll keep trying!!