Notejoy does not offer an automated way to merge Notejoy accounts. However, there are two alternative approaches:
If you are trying to move your account to another email address, you can simply change your existing account's email address. You can do this from Account Settings. Once you change the email address, even Google login will work for this new email.
Move Libraries & Notebooks
You can leverage Notejoy's team library sharing capabilities to transfer libraries and notebooks to another account.
If you already have a team library that you want to transfer to another account, simply invite the new account as a team member and give them admin permission. Then go to the new account and you can remove the old user as the previous admin. The team library has now been transferred to another account.
If you don't already have a team library, just create one and move the notebooks that you wish to transfer into that team library by dragging and dropping them into the team library. Then you can proceed with the above directions on how to transfer team library ownership. From the new account, you can then drag and drop those notebooks to another team or personal library if you like.