i have three different ideas about them being at heavens arena 

1) that hayami knows just the basics of nen so hisoka helps her

2) she doesn't know anything about nen and learns with gon and killua

3) hayami knows everything about nen and spends all her time with hisoka

 with any of the three you pick; when  she uses nen it her clothes  changes along with hair style/color for the kind she's using at the time 

so you can pick because i don't know which one is better for the story also i really i hope you like my story if not i'm sorry but i am letting you give suggestions so its more to your liking