Merging Photos Libraries (1 formerly iPhoto library)

Hi, I found on here that the way to convert iPhoto to Photos was to hold down the alt/option key and select the library. I have two--one on an external drive that is my old iPhoto that will open in the now Photos app and goes through a certain date, which I believe I successfully converted and then the native Photos app that has my album from when the system upgraded/what matches on my phone. Can I combine them in some way? When I open the photos app and press the alt/option key, I get this message. I'd like to combine.