I added my contacts before I installed WhatsApp. And these contacts appeared in WA without any problems.

After I installed WA, I noticed that (at least) one contact was missing (got not synced from my N9 correctly). So I added that contact manually in the People app but that contact did not appear in WA.
After a reboot of my Jolla, the contact appeared in WA contacts list.

I think, the local contacts get synced at boot time with the phone book in Android.