As far as I know, there is no "K2 smartforms SDK".

There is the "K2 SmartForms Dokumentation (C:Program Files (x86)K2 blackpearlHelpK2SmartForms.chm)

I searched for "sample" inside and found this:
ms-its:C:Program Files (x86)K2 blackpearlHelpK2SmartForms.chm::/Creating a Custom User Task.html

But this is about the workflow designer, not smartform controls

You want to create a custum control, which you can use in the forms and views designer of smartforms.

Here is a solution Andrew Murphy build for Lync control integration, with code:
http://www.k2underground.com/groups/lync_presence_and_contact_search_controls/default.aspx

He also has a video on youtube about it: http://www.youtube.com/watch?v=ow_448qgXA8

Maybe this helps.

Update: There is also a "Creating a Custom Control" in the Help file.
ms-its:C:Program Files (x86)K2 blackpearlHelpK2SmartForms.chm::/Creating_a_Custom_Control.html

We have a control SDK that allows for custom controls to be
created and would recommend having a look at that. browse to http://help.k2.com under the K2 smartforms menu
you will find a documentation section download the Creating a custom control
(PDF). The community has also created  a couple of blackmarket projects
related to custom controls so feel free to download those as examples of what
you can create: http://www.k2underground.com/groups/default.aspx?GroupID=66

Also check out the VS templates for creating custom controls: http://www.k2underground.com/groups/visual_studio_project_templates_for_k2_blackpearl_extensions/default.aspx