qemu-img create -f qcow2 foo.qcow2 64m
qemu-img bitmap --add foo.qcow2 mybitmap

This creates a bitmap extension header like this (starting at 0x1f8)

000001f0  00 00 00 00 00 00 00 00  23 85 28 75 00 00 00 18  |........#.(u....|
00000200  00 00 00 01 00 00 00 00  00 00 00 00 00 00 00 20  |............... |
00000210  00 00 00 00 00 05 00 00                           |........        |

And a bitmap table that looks like this:

00050000  00 00 00 00 00 04 00 00  00 00 00 01 00 00 00 02  |................|
00050010  01 10 00 08 00 00 00 00  6d 79 62 69 74 6d 61 70  |........mybitmap|

I modified the bitmap table to add eight bytes of bad data:

00050000  00 00 00 00 00 04 00 00  00 00 00 01 00 00 00 02  |................|
00050010  01 10 00 08 00 00 00 08  62 61 64 64 61 74 61 21  |........baddata!|
00050020  6d 79 62 69 74 6d 61 70                           |mybitmap|

And modified the header accordingly to add eight bytes to the table (0x20f := 0x28):

000001f0  00 00 00 00 00 00 00 00  23 85 28 75 00 00 00 18  |........#.(u....|
00000200  00 00 00 01 00 00 00 00  00 00 00 00 00 00 00 28  |...............(|
00000210  00 00 00 00 00 05 00 00                           |........        |