ROFL files cannot be imported directly to RiftGG as they are not a video file. 

However, you can download your .webM files and import the to RiftGG. You will find them in: C:\Users\%User_name%\Documents\League of Legends\Highlights. 

Tip by Rift:

Did you know that video editing programs like Camtasia do not accept .webM?? 

You Can use RiftAnalyst to convert the webM files to MP4 and then, open them with Camtasia.