This is a very long process it looks like... will we have to do the same thing once dwallorde has the file in his download, or no? The reason I ask is because i have to get my server host to do all of this for me, and they tend to say no when it's too complicated of a process. So, if it will be a more simple update once it's all in the WotW file, i will just wait.@qixxy,
Until Dwallorde adds the file to his download, you can get a copy of it here.
https://7daystodie.com/forums/showthread.php?34533-Backpack-Expansion
This will work, just pick the one that has the correct size, or at least is larger. Only use the dedicated DLL though.

Alternately.... can I install the 5.1.1 update now and then add the Bigger Backpack addition later, or will that break everything?