Can an apostrophe be added to the word Phoenix?

No. You can, however, add an apostrophe -s: Phoenix's.

The best strategy of all is to add 'es', giving you "Phoenixes"

That's English

---

Adding apostrophe s indicates that something belongs to Phoenix. Apostrophes aren't used to create plurals.

In English, -es is added to Words to create plurals, not possessives.