Gfrktyl

dumpe2fs -b /dev/sdd1
can dump blocks that are known to be bad by ext4.

How do I manually add a block to that list without redoing mkfs ?

From man 8 e2fsck:

-l filename

Add the block numbers listed in the file specified by filename to the list of bad blocks. The format of this file is the same as the one generated by the badblocks(8) program. Note that the block numbers are based on the blocksize of the filesystem. Hence, badblocks(8) must be given the blocksize of the filesystem in order to obtain correct results. As a result, it is much simpler and safer to use the -c option to e2fsck, since it will assure that the correct parameters are passed to the badblocks program.

-L filename

Set the bad blocks list to be the list of blocks specified by filename. (This option is the same as the -l option, except the bad blocks list is cleared before the blocks listed in the file are added to the bad blocks list.)

I've seen few output files from badblocks that were not empty, so I can tell the general format is simple: one decimal number per line. Numbering starts from 0 but as far as I know e2fsck -l won't accept 0 and will complain about few initial blocks (where some crucial filesystem metadata exist).

Example file content:

12345

678900

My tests were limited. Try the solution on a scratch monkey first.