As others have said, it's great that you want to embrace this side of yourself more. Before you commit to buying anything and making your dreams a reality, you'll really need to consider some of the ins and outs of your living/roommate situation.
Luckily, I've been fortunate enough to have been living on my own since 19. My grades were high enough that I got a residence space all to myself in my first year of University, no roommate at all and when I transferred schools and changed majors I had enough money squared away from a part time job to rent my first apartment. I'm currently in apartment #2, a bachelor pad, all my own. I haven't had to deal with pesky roommates , University rumour mills or school run mail rooms. My packages are held at the apartment mail room for me to claim, if they are too large, they are held at the local UPS or post office for me to pick up and the only people who enter my living space are friends who are well aware of my AB side, I don't need to worry about a diaper being balled up in the trash or a pacifier lying around the room.
You are presumably going to be staying on campus. In first year, word spreads fast about everything, so even though you want to explore this side of yourself, you'll likely need to keep your exploration on the down low, otherwise you'll risk others finding out.
I would suggest before you go about procuring baby items and diapers that you take stock of your dorm room, for instance, is it a shared dorm space? If it is, you'll really have to tread lightly and look for hiding places so this interest isn't found out (your lockbox idea sounds great), you also need to be considerate and mindful of your roommate. If you have a shared living space, but separate rooms with their own doors and locks, then you have a little more leeway to explore and indulge and the privacy necessary to keep the essentials around.
Also, I assume you would be ordering AB items through the mail? If so, look into how your school handles packages. I have friends from my recent University who live on campus and there have been waves of package theft there because the mail room is to small to keep a hold of all student packages and if said packages are on the smaller side, they're taken out of the mail room and left out in the dormitory hallways where any rowdy student can get a hold of them. If your school has a designated mail room, that'll make claiming whatever items you order easier.
Lastly and most importantly, the roommate situation. I'm with you, you certainly don't want to reveal this side of yourself to your roommate. Maybe if they turn out to be a half decent person you can reveal this to them later on, but for now, I'd take an approach of complete secrecy. Also, get to know your roommate's schedule and their habits, this will help you plan out when it is safe to wear and indulge. Chances are you might even have to wait for your roommate to go home on the weekends (many first years do this) before you can wear and regress open and freely.
Your desire to explore your AB side after 18 years of living under your parents roof is understandable, but wearing and indulging at University, from what I have heard from other AB's is far from a cake walk. Living the University life can be difficult for the most average of Joe's let alone for us AB's. If you have your own room with a lockable door, then I'd say you have the space and privacy necessary to indulge without fear of being found out, but I know that most dormitory set-ups are shared spaces, so you'll likely have to hide your items and indulge in secret, only when time allows and when your roommate isn't around. If you hit it off with your roommate and can gauge whether or not they're accepting, then maybe down the line you can confide in them, but that depends on who your roommate is and how comfortable you would be with that.
In any event, I wish you luck and I hope you can find the time, space, privacy and confidence to explore safely. Also remember, save the diapers and the baby time for when you have a handle on your studies