Is it possible to perform layout only on a diagram subpart ?

My issue is the following :
I add new node by code and I need to place this node.


Yes, but I suspect what you are really asking for is incremental layout that takes all of the existing node positions into account.

That’s possible with ForceDirectedLayout (with the “IsFixed” member), but not with the other layouts. The reason is that adding one node or one link may cause all of the nodes to need to move.