I think he means adding 400 km onto an existing 600 km BRM calendar event. For that you treat the total event as a 1000 km DIY, so you get 1000/13.3 = ~75 hours from start to finish. You still have to stick to the original timings of the calendar event, of course.
Indeed, and it can be very useful.
Consider doing a 200km ECE to the start of a calendar 600 (which, for the sake of argument, is BRM so 40h) and then 200km ECE home afterwards.
If the calendar event starts at 6am on a Saturday then you could work backwards as follows:-
200km ECE to the start at 13.3kph (because you're doing a 1000km ride). So that gives you 15h to do that 200km to the start.
Saturday 6am - 15h = Friday 3pm, so:-
A Friday 3pm start and 75h (3d 3h) time limit for a 1000km ride you have to finish the whole thing by Monday 6pm.
Friday 3pm start a 200km ride to the start of the calendar event. Knock that off in 12h or so and get a 2h nap. Faster = more sleep.
Saturday 6am start the 600km calendar ride, adhering to the calendar ride time control time limits/etc.
Sunday 6pm (for example), finish the 600km calendar ride.
That then gives you 24h to sleep and then knock off the 200km ECE leg at the end.
If you want you can front load the ride a bit more by starting later than Friday 3pm which pushes the end time later too, but you can't start any earlier than 3pm (if the calendar ride starts at 6am on a Saturday and the whole ride is a nominal 1000km with 200km ECE legs either side) as you'd be out of time at the start of the calendar ride.
But finishing a 600km in 36h (having already done a 200 to the start) and then having 24h to sleep and then knock off a final 200km is quite a nice way of doing it. Even if you push the 600 to the 40h time limit you've still got 20h to sleep and then do the final 200.
One day I'll do the 200km from SW15 to the start of the BCM, ride the BCM and then ride home again to make it into a 1000. One day.