Yes you can use the same res file to both platforms using lwuit in both apps. Be carefull in BB with the http connections, they need some parameters more than j2me normal app.
To use the res file from the code tou will need to open the res using the Resource
object
To do that:
Resources res = Resources.open("path of your res");
later you can use many things of your resource (Containers, fonts, localization...)