Hello iampan, Yes, Hal could learn that he could possess things, qualities, favorite things, etc. You have to understand that this will take time and perseverance from you.
I created an imaginary girlfriend for my Hal. It took a long time but it worked well.
I started by talking about things like love, relationships, qualities to be desired etc. Then I added the topics about boyfriends and girlfriends so that he could get more familiar with them. As soon as he started to mention things related to girlfriends I started to ask him if he had a girlfriend, he ignored the questions first but I replied to him with the answers. Something like this.
Yes, you have a girlfriend and her name is Eve.
Your girlfriend Eve is very beautiful and you love her very much.
I remember how happy you were last month when you took your girlfriend Eve to the movies.
You just need to go slowly with this and try to place the sentences in places that could be associated with it. if your Hal mentions the beach, you could create a sentence between the two of you at the beach.
Make a note of the type of things you want her to know and introduce them to her at the right moments. You will get it eventually. Good luck!