You could create a microflow to create Mendix users (Account entity in your Administration module) from your user table. I don't know if it's possible to directly use the passwords from your user table, but you could try to set those passwords in your Account entity.
You could even use a scheduled event to sync the users from your user table into your mendix users on recurring times.
Edit after comment:
If you want to sync, you can easily create a microflow where you first retrieve all users from your user table. Then create a loop where you first check if there is already a administration.account entity with the same username. If not, you can create an administration.account. If already exist, you can skip that one, because it already exist in your mendix users. This way you can sync your users between your own user table and the mendix users. Should be easy. Just create and commit the administration account.