[3.1.4 modification] modify calendar library, add extra column before calendar

i love the calendar library, but i just miss a single feature. i need to add a column before or after the calendar for (for example) the weeknumber or other thing relevant for that week.
Is there any way to modify the calendar library (by copying it to my own library folder and modify it) to allow for an extra column before the first or after the last column with a way to add data into it ofcourse....