First of all, i don't know if this widget is a launcher only widget.
Gravity Box for example has a option called "Allow any widget" in Lockscreen options.
It allows those widgets on Lockscreen that can normally be used within launcher only.
But what you can try first ist go to Settings > Security > Enable Widgets
Now lock your Phone und swipe on the Lockscreen to the right, tap on + and add your widget.
1. Don't use the lockscreen icon to swipe
2. Swipe directly from the left edge to right