Is there a way to add me as joint owner of father's bank account given our situation (see details)?

My father had been declining noticeably, both physically and mentally, over the last few years, but had always refused help. He was then hospitalized for a month and I found out about many overdue bills which needed to be paid. At this time he gave me access to his checking account w/ debit card. Based on whatever clues he could offer and access to his PC I figured out his account names or PWs for the utilities he had once been paying (manual payments monthly, never autopay, hence the past due issues). So I got all of his accounts auto paying with whatever method he had on file.

FF to this year and his mental and physical states have dropped hard and fast, like falling off a cliff. He then suffered a stroke and other medical issues which has left him in a wheel chair and virtually unable to communicate, certainly not at a level that legal proceedings would require. As you might guess from the above description nobody has been granted PoA for any aspect of his care or finances. All I have is this bank account. We have started the slow process of petitioning conservatorship where we will eventually be able to apply his savings to care for him, but until then we can't convert the savings (retirement, stocks, etc) into cash to do so. Not all of this is pertinent but I thought I'd tell more of the story.

My question has to do with a recent issue I noticed with his checking account. Two ACH withdrawals have occurred this month that I had not authorized, and dad couldn't physically have done it. Both are withdrawals for utilities. Utilities dad uses or has used. The first one was to the TV/cable company, but that account had been shut down and paid off months ago. I can't even access it online anymore since they discontinue access immediately upon account closing. The other, very recent withdrawal is to the electric company. I have access and no such payment has been applied. That account is paid in full and on autopay. I called both utilities and they have no record, both telling me to contact my bank.

So, I figured out with fair certainty that someone has found dad's first name, last name, account number, and routing number. The same four things that are on any check. Or available if an ACH transfer was done by phone or internet and the data stolen. My guess is that someone received a paper check from him in the somewhat recent past and is paying their utilities with his bank info. It really is that easy. I know because I've set up one time payments and recurring auto payments with only those four pieces of info. This is scary. I've contacted the bank and had to pretend to be him, which I hate, but it was the only recourse to start an investigation. Whether they refund the money or not, a process that can take up to 90 days, this thief or anyone else who has this easy to obtain info could do the same thing again. The bank's suggestion is to change the account number. Dad is not capable and I can't legally do this. It must be done in-branch.

Does anyone know of a legal method, given these extenuating circumstances, to apply to the bank as a joint bank account owner, or act as proxy without any PoA? I would like to get this done before the months and months of conservatorship petition process has completed. The account could continue to be hacked until I do something.

I was thinking of one far fetched possibility; if a kind executive soul at the bank was willing to arrange a notarized type of meeting at dad's memory care center. Dad can answer some questions properly but gets distracted due to the dementia. Maybe I could be granted joint ownership that way just like if we both were to go into a branch. I can account for every expenditure I have made since he originally granted me access two years ago, so maybe an overwhelming amount of evidence and their compassion could get this done.

Are there express legal forms and proceedings for this situation, short of achieving PoA?