If you have a mix of one or more iPhoto libraries and Photos libraries that you would like to merge together, there is no way to directly merge an iPhoto library into a Photos library, or vice versa. What you can do instead is to first migrate the iPhoto libraries that you want to merge over to Photos libraries. You can then use PowerPhotos to merge the resulting Photos libraries together with your existing Photos libraries.
When not to merge an iPhoto library with your Photos library
There is one common situation in which many users believe they need to perform a merge of an iPhoto library and a Photos library, but is usually not necessary. If you have an iPhoto library that you already migrated over to Photos, the iPhoto library will remain on the hard drive after performing the migration. This will result in an iPhoto library and a Photos library appearing side by side in your Pictures folder.
It can be easy to forget that this happened, come across the iPhoto library later on, think that it has different content from the Photos library, and thus needs to be merged. However, since the Photos library starts out with everything that was already in iPhoto before the migration, performing a merge will usually just be a waste of time, since no new content will come in from the iPhoto library that wasn’t already in the Photos library.
The only exception to this is if you migrated to Photos, but then continued using iPhoto for some time after performing the initial migration. Only in this case, where you have added new content to the iPhoto library after the migration, should a merge be necessary.