Could anxiety cause attention problems as severe as ADD and is venlafaxine suitable for such combination of Anxiety and ADD?

Yes possibly. Its important to have a diagnostic evaluation by a psychologist or psychiatrist (that is good at differential diagnosis) to first evaluate what you have, before deciding on a treatment plan. Anxiety can cause problems in attention, focusing, concentration, etc. Depression can also come with these symptoms. ADD/ADHD can occur with anxiety and/or depression. Treatment follows the diagnosis.
Yes. Anxiety can certainly cause severe attention problems. Venlafaxine can often treat anxiety, and it can also treat ADD.