From b2evolution manual
This manual page is outdated. NEW PAGE: Hack in b2evolution v5+.
 What is a Hack?
A hack is a way to change or extend the functionality of b2evolution by editing the underlying code. Hacks are often used to achieve a specific change to the way b2evo works when a plugin is not available.
Hacks can be very effective but they have one large drawback. Anytime you upgrade to a new version of b2evolution you will most likely need to overwrite the files you have hacked, thus your changes will be lost. You will have to re-edit the code to restore your hack.
 Find hacks
There are two main sources of hacks for b2evolution. Some of the more popular hacks have been gathered together at the Plugins Repository. You can also find hacks by looking through the b2evo Forums in the "Plugins & Hacks" section.
 Request a hack
If you would like to request a hack the forums are the place to do so.
 Drawback with hacks
Keep in mind that hacks are often specific to a particular version of b2evolution. It's a good idea to do some research and make sure the hack you are trying to implement works for the current version you have.
Since Phoenix, you should use Plugins instead of Hacks, that allows far easier upgrading and more flexibility for both plugin authors and users. It's probable that there are plugin hooks missing - but the solution is simple: please just request them.