You did not specifically state it in your OP, but I assumed you were referring to a 2020 week (because those are the only weeks with no availability on the DVC side). As several have stated, if your reservation is in 2021, yes hotel guests can rebook and add tickets. If however, the hotel stay, DVC stay, or off property stay is in 2020, no one can buy tickets yet. The only word from Disney on 2020 tickets is later this Summer. After this Sunday anyone with a ticket (with resort, with AP, or just plain ol' with a ticket) can reserve a park. Once Disney gets a chance to see what availability looks like at that point, they may release tickets (for specific dates that still have park availability?). We do not know what assumptions, if any, they have made as to when they go from 30% to additional capacity. So who knows, as capacity changes 6 months from now, additional ticket could get released. Unfortunately, no firm plans communicated and I am guessing, no firm plans have been set since no one knows what is going to happen in the coming months. The best we can do is make plans based on the current situation and each persons comfort level. To the original question, who will get preference?
For 2021 stays - here was the batting order
June 24th - People with existing cash reservations could modify to add tickets
June 24th - People could book new cash hotel packages that included tickets (although the wording in the announcement indicated this would happen on the 28th)
June 28th - People can buy tickets for 2021 (we believe this will not include new AP's)
June 28th - DVC room only can add tickets - this is really not a unique category - We are just buying tickets in the standard 28th release
With 2020 tickets you have the same basic 4 groups - Add a new room package, Add tickets to an existing hotel room, Buy tickets, and add tickets to a DVC room (once again - could be the same as just buy tickets). Once they roll out the announcement, we will know where we stand in the pecking order. Will these all be one day, or will they stagger them. Based on the 2021 rollout, we better hope for one big release. If they stagger them, the precedent is DVC guests without 2020 tickets will not be in the first group getting a chance.