Emil,

Here are two blogs that I wrote on the topic.  Basically you need to write a custom SmartObject Service that will wrap your custom DLL.  We also wrote about this in the K2 Wrox book...

http://www.k2distillery.com/2007/11/create-custom-blackpearl-smartobject.html

http://www.k2distillery.com/2008/09/creating-custom-smartobjects-part-2.html

Jason

japergis:

Basically you need to write a custom SmartObject Service that will wrap your custom DLL.

Japergis is right, the current way I know is:

- implement a ServiceObject in a ServiceAssembly 

- register the service assembly with "BrokerManagement.exe" tool (in the k2blackpearlServiceBroker)

- create a service instance with the same tool

- design a SmartObject and add custom method calls (in advanced wizzard), pick the service instance from the toolbox

- deploy the SmartObject

- test the SmartObject with "SmartObject Service Tester.exe"

I'm currently doing this to obtain destination set for workflow activites from a wcf service

Let us know how it works for you