How do I import iphoto library to Photos?

Launch Photos with the Option key held down, click on Other Library, and choose the iPhoto one.

(189388)

When you do that, your iPhoto library will be upgraded for use with Photos. It would be wise to take a backup of it before importing, if you don't already have one.

Note also - that you will get a second library created containing just your iPhoto images - it won't transfer the images from the iPhoto library into your current library.

If you then want to merge the two libraries there are a number of ways you can do it.