How can I add the standard Asana Field for priorities on an existing project (I am on free version). I don’t see the field as an option under Customize.
Welcome to the Asana Community Forum @anon25763229 
Have a look here I can't see Priority - #2 by Phil_Seeman
You might consider using tags instead.
Another option for example is Ducalis.io • Asana, explained in this post.
Thanks Andrea. Ducalis looks a bit too advanced for me at this point, but definitely want to determine some good practices for using tags and categories to:
–bring visibility across the team to what members are working on now
–manage task priority
–find the right task for discussion posts around a complex task
I see. Yes so the priority custom field should actually be available in the free tier as well.
You should be able to add it via the customize menu.
Alternatively the tags option works too. Once you have tags added when you click on them you can also see all tasks with that tag.
This appears to no longer be true–after my trial was over yesterday, the Priority field is now locked behind the premium tier.