I've found the answer (by further examining the Sublime 2 config files structure):
I was to open
~/.config/sublime-text-2/Packages/Scala/Scala.tmLanguage
And edit it to add sbt
(the extension of files I want to be opened as Scala code files) to the array after the fileTypes
key:
<dict>
<key>bundleUUID</key>
<string>452017E8-0065-49EF-AB9D-7849B27D9367</string>
<key>fileTypes</key>
<array>
<string>scala</string>
<string>sbt</string>
<array>
...
PS: May there be a better way, something like a right place to put my customizations (insted of modifying packages themselves), I'd still like to know.