Hi,
I think this is possible, but it needs to customize the calendar functionality.
The calendar javascript object allows for this, but I'm not sure about K2 saving process whether it supports this feature.
In the calendar setup, you can use 'multiple:true', to select more than one date, but I have not tested it yet with K2
Multiple Extra Fields Groups for K2
AutoMeta for K2
Chained Fields for K2
More K2 Extensions In My
Extensions Store