In fact, your idea is pretty good. So your users have a "default value" that will speed up the process!
Nevertheless, I recommend you to post on the form (just below the box) a message of the type "We filled it for you, taking your account information" to make them understand why things are done (BONUS if you can have green color, confirming that everything is fine).
Nevertheless, I would recommend testing whether such emotions cause bad emotions (I mean, they could NOT like at all the fact that you fill in this information … who knows?). Pre-test the idea with some people (with the help of qualitative studies), then with more users in a "beta feature" before finally implementing it.
Plus, I do not have enough information to give you details. But you should ask yourself if it is really worth asking them this if they are already connected? I do not see any type of business needing to "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 it automatically (without asking them to enter their email address and their word password, but allowing them to do the minimum required). This is not a "best solution" because I do not know enough, but it is one of the solutions that you can implement.
I wish you the best!