I can't add the word 神さま to my custom list. I then get two options; to add it to the system as either a word or a sentence. Neither works, because it says it's in the system already.

I have two words involving the kanji 神 from the Genki textbook for homework: 神さま and 神社. I would prefer if I could have those two exact words in my list.

Current workaround is adding just 神 and 神社.

You should be able to add 神様 instead. Is this listed in your textbook as 神さま, though?

Genki I does indeed list "Kamisama" as 神さま. 様 is a kanji which my class has not encountered yet, and it will not appear on our tests, so I have no intention of cramming it at the moment.

You should be able to add this word entry as 神さま now, thanks for the your report!