Can photos be added to a shared iCloud photo stream album even after it has reached 5000 photos?
First of all, to remove any confusion, iCloud's My Photo Stream, and iCloud Photo Sharing are two different things, and will behave differently.
iCloud Photo Sharing
From the FAQ:

How many photos and videos can be stored in a shared album?
A shared album can hold a maximum of 5000 photos and videos combined. When you reach your limit, you must delete some photos or videos before adding new ones.


My Photo Stream Limits

Uploads to My Photo Stream per month: 25,000 photos

Also note that photos are automatically deleted from your photostream after 30 days.