Dear Pan 123,
Welcome back to the Wefugees Community and thank you for your question.
If someone holds a Duldung and wants to relocate within Germany, he/she will need to hand in a so-called "Umverteilungsantrag" at the local immigration office (Ausländerbehörde). It is not easy to get the permission at the end and both immigration offices (the sending and the receiving) need to approve. However, a family unification might be a strong reason.
I hope this helps and all the best,