Hi,
change into passive : " Enter by this door" (A notice)
Is it all right if I write : Let this door be entered?
No, that's not right at all.
You'd need to say something like 'This room is to be entered by this door',
or 'Let this room be entered by this door'. But this is awkward and terrible, terrible style. Native speakers would just laugh at this.
Why do you want to use passive? Is it just some grammar exercise?
Most doors would just say 'Enter'.
Best wishes, Clive