HTMLawed exception for class="elgg-lightbox"

Hello everybody , i know in HTMLawed "class" attributes is denied , but is it possible to make a exception?

For some reason i want  class="elgg-lightbox"   will not be denied, 

so when my user input class="something" it will be denied by htmlawed but when input class="elgg-lightbox" it will not be denied

is it possible ? how ?