Hi Pablo,
Thanks for contacting Syncfusion support.
We have checked your requirement from our side. In our current implementation we do not have direct support for clear button and keep the original buttons intact. We have considered your requirement "Nullable picker with extra button in footer named Clear" as a feature request.
It will be available in any of our upcoming release. Please check our website periodically to know the status of the features that are implemented.
We have created a sample based on your requirement. In this sample we have created custom footer with clear button, Ok and Cancel buttons similar to original buttons. Please find the sample from below link.
Sample: http://www.syncfusion.com/downloads/support/forum/136833/ze/GettingStartedPicker-1401986629
Please let us know if you have any other queries.
Regards,
Vinnalan K A.