Importing an iPhoto library in Photos

Not sure if this will solve your problem but it might be a good starting point.

Try rebuilding the iPhoto library. You can do this by holding down the Command and Option keys while opening iPhoto. You will then need to choose the rebuild options. I think it would probably pay to select all of them. iPhoto will then work away for a long time and rebuild the library.

There is an Apple support web page that details this process further. I would normally say that you should keep a backup before making any changes like this but I guess you still have the copy of the library on the old iMac

If the rebuild doesn't help then I'm not really sure what you can do.

Open the old library directly by holding down the [option] key while starting iPhoto. iPhoto will then allow you to browse to the library location (use the "Other Library" button) and should update it automatically to iPhoto '11 format. You may want to back it up first

Close iPhoto and restart holding [option] once again, make sure the active library is your current (new) one. Try reimporting the old library now. It might still be greyed out in the file picker but double click it anyway, it should import.