This is not possible, but there is an easy work-around. Import the 2011 bundle into the corresponding 2011 project (LPU), open the 2011 project in 2015 to convert it to the 2015 format. Now you can leverage between the 2015 projects.
This is not possible, but there is an easy work-around. Import the 2011 bundle into the corresponding 2011 project (LPU), open the 2011 project in 2015 to convert it to the 2015 format. Now you can leverage between the 2015 projects.