I wanted to do the same, and this site generates an arbitrary weekly event.
Arbitrary week number generator for ical compatible calendars
If you require week numbers for you calendar but your week one does
not on the first week of the year you have some problems. Google
Calendar and several other gizmos offer week numbers but they all
start on the first week in January. This php script can help.
Instructions: enter a name for your calendar, choose the date to match
the first day of the first week for your week one and press the do it
button. The script will take you to a directory where your week number
calendar is stored. Save this to your computer and then import it into
your calendar. In google calendars I recommend creating a new calendar
called week numbers and then importing the events to this (it makes it
easier to delete the dates if you go wrong and makes it easy to change
colours etc etc). the import function is via the "gear" that appears
top right: calendar settings:calendars:import calendar. Remeber to
import to "week numbers" for ease of use.
Beware, create an extra calendar to import the ical to, because it might be difficult to clean it if you make some mistake.