Panels call custom function
It is, but requires a bit of coding. You need to create a ctools content type plugin. I had the same issue a few months back, and found this to be a reasonably good guide for writing you r own content type:

http://shellmultimedia.com/articles/creating-content-type-ctools-panels-3

There are also a couple other options:


Expose the data your function is providing in a block. Blocks are automatically supported by panels. 
You could (likely) use custom PHP in a panels pane. (highly discouraged)