How do you mark favourites photos from synced photo albums?

I have 5000 photos which I synced to my Iphone when I first switched from Android to Iphone 6 months ago.

I now would like to select a few of these pictures and mark them favourites, but IOS is not letting me doing it as the favourite icon is not available for synced albums.

How can I mark them as favourites without the need for duplicating each picture?

The downside with duplicating pictures it is that the newly created picture appears listed on todays date rather than on the year/month it was taken, which is a no-no for me.

Please let me know

Many Thanks

Luca