Is there any way of adding custom icons to buttons?

You should inspect the plugins from @Grum999, especially Buli Commander. He found a way to add his own icons to Krita, but it is a very laborious way, and I was unable to fully grasp the process.

Michelist

Add/Edit: As you can see in this screenshot, he created a menu entry for Buli Commander and added an own icon to it.

In this posting you can find a description how to patch the last version of the plugin to make it work again. Alternatively, you can download it already patched from my cloud:

It is also found a few postings below @Grum999’s description and offers another download option if you should have issues with downloads from MEGA:

Michelist