Actually, your idea is pretty good, this way your users have a "default" that will speed up the process!
Still, I recommend that you show some message on the form (just below the box) with something like "We fill it out for you, taking your account information" so that they understand why things are filled (BONUS if you can have some green color, confirming that everything is fine).
Still, I would recommend testing if there are bad emotions for this type of "intrusion" (ie, they would not like the fact that you fill in that information … Who knows?). Perform a pretest of the idea with some people (using qualitative studies), and then with more users in a "Beta Function" before finally implementing it.
Also, I do not have enough information to give you details. But you should ask yourself if it is really useful to ask them if they are already connected. I do not see any type of business that needs to "sub-create an account". Not to mention that if the results of your "Beta Feature" study show that people are not comfortable with this, you should probably implement this automatically (without asking them to complete the email address and password, but allow them to do the minimum required). This is not a "best solution", since I do not know enough, but it is one of the solutions that I could implement.
I wish you the best!