At a guess, here's what you'd need to do:
You''ll need to make a copy of the "Plain Text" syntax type first. You can then make settings for your new syntax type with custom default character encoding, and assign certain file extensions to be opened with the new syntax type...
In your sublime profile folder (say c:\users\xxx\AppData\Roaming\Sublime Text 2\Packages) make a copy of the "Text" folder (this is the folder for "Plain Text" syntax type) and rename the folder copy to (say) "P Text".
Next open up a .p file and click bottom right to change syntax (where it says "Plain Text"). The big pop-up menu, choose "open all with current extension as...", and choose your new syntax type "P Text"
Finally, with a .p file open (and syntax as "P Text"), choose menu "Prefs..Settings more..Syntax specific".
Add the following lines to the empty syntax file and save it:
"default_encoding": "Cyrillic (KOI8-R)"
Give it a try and post back if it works!