Yes, you can.
Enrolment after the course start is not possible. You have to wait till enrolment period for next term opens. Note that there might be serious issues with migration authorities so please plan your entry carefully beforehand.
No, you can only attend courses with an admission letter of a Viennese university that states one or more supplementary examinations. More information: www.vwu.at
Students from certain countries, students that are also asylum seekers, recognized refugees and students with subsidiary protection in Austria, as well as students with supplementary tests from another subject if they already have advanced German language skills.