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.