Yes, you can have multiple Photo libraries.
If you want to keep this old iPhoto library separate from your current library, all you need to do is hold the option key down when launching Photos, and it will ask which library you want to use. Navigate over to the older one, and if it is openable directly then Photos will import those pictures into a new Photos library (be aware that this will take up a lot of extra space depending on the number of photos in the imported library).
If the old iPhoto library is not "visible" as a Photos library, you also have the option to create a new one (and store it on whatever drive you want). The new one will have zero pictures in it, but you can then import from the File menu in Photos and point it to the old iPhone library.
By default, Photos will then open that new library every time you launch it. To change libraries, launch Photos holding the option key, and it will again ask which library you want to open.
