You can do this in a Set Data Values extension.
If the variable you want to uppercase is page_type then you would select page_type in the "Set:" field. Select JS Code in the "To:" field.Then type "b.page_type.toUpperCase();" (Quotes not needed) in the text box on the right.
So your extension would be outlined like so:
Set: page_type To: JS Code b.page_type.toUpperCase();
Then you can add a condition if you only want this to take place at certain times.
Hope this helps