Yes !! I found a way !
I just had to create a dummy menu with k2 category type, and select all categories, in the config. And back to the breadcrumb module, I assigned it to this dummy menu.
hth

I am trying the technique above, but the screen will not let me put anything in the categories field.
Am I using the wrong type?

Remember K2 is meant to be a content replacement for J! articles/cats/sections.
It is possible to run them together, but you must consider that you would not be able to run Articles or Items on the same page unless in modules.

J! breadcrumbs will only look for J! content in J! sections or cats, K2 will be the same but for K2 content only.
So you would not have good results running K2 BC with J! content.

Unless the breacrumb module is hacked to look at either J! or K2 structure, then you would have to assign the BC module to the appropriate menus.
You will also have to ensure that the appropriate menus are point to the right format content.

Recently a user had upgrade a whole site in K2, but complained that they could not get K2 content displayed.
This was due to all menus pointing to J! content.

It is easy, but a little thought is needed to apply it.

I do need to ask, why not use just K2? instead of both?

You have to choose the category type in "item list"

John Langlois said:I am trying the technique above, but the screen will not let me put anything in the categories field. Am I using the wrong type?