To open the iPhoto Library in Photos for Mac launch the Photos.app while holding down the options key ⌥, until the library chooser dialog appears. In this dialog select the library on your external SSD.
This will work, if your external volume is compatible with Photos for Mac.
- The file system format of the volume needs to be MacOS Extended (Journaled) or APFS.
- The volume must not have been used for Time machine backups.
- The volume needs to be locally mounted, not shared on the network. If your external volume is a NAS or connected to Airport or similar, plug it directly into your Mac while you try to convert the iPhoto Library to t Photos library.
See also: Updating from iPhoto to Photos for OS X - Apple Support