Can I merge another library to the system library using power photos ?
I used PowerPhotos predecessor iPhoto Library Manager to merge libraries some years back. I would probably make a backup and turn off iCloud sync until you are happy with the merge.
BTW, my license for IPLM allowed me to register PowerPhotos. Great company, thanks Brian.
This is from their website. The example they use shows merging the system library (iCloud library) with another library. Go to their website and click on the graphic, shown below, to see the example.
— Great advice from @WayneG, above.
Can I just make a duplicate copy as the backup.
Sure. I frequently just duplicate and rename files, prior to major “operations”. If possible I like to put them on a different volume or at least in a different folder to prevent mixups.