The class java.lang.Long has a static method toString(long i, int radix) which will convert a Long into a string representation of another base. "Radix" means the same thing as "base" in this context.
val myLong = 25000L
val longInBase36:String = java.lang.Long.toString(myLong, 36)
Scala will treat your scala Long value as a java.lang.Long when necessary, so you can always look for methods in the Java API documentation when necessary. See: http://docs.oracle.com/javase/6/docs/api/java/lang/Long.html