Now I have one Schema like that

A–(C,D,E…)

A is the root node
and C,D,E are its direct child nodes.

Now I want to insert an new node “B” like this:

A–B–(C,D,E…)

The way now I use is something complex: rebuilde it!
Is there any way I can do it easily?
(In SchemaEditer or in XMLSPY)

You can make certain modifications to the schema without having to go the process of unload/change schema/reload. However if you want to insert a new node into the schema in the middle of a sequence of nodes then you will have to go through that process. The reason is simple: if you could modify the schema in this way, all existing instances are instantly invalid.

If you want to insert new instances of ‘A’ with nodes ‘B’ and you really don’t care about validating it you can change the schema to open content which effectively allows instances with additional unspecified nodes to be stored.

This might be a workaround:

Select the complex element “A” ( I suppose it must be one, since it has 3 element definitions in its content model) within Tamino Schema Editor, create a new child node B.
Afterwards cut each element definition C, D, E using CTRL+X and paste them beneath the new child element B. Child element B must be selected, then just press CTRL+V.
The order of element definitions on the same level can be changed using the menu entries: Edit->Move up, Move down.

Improvements are planned for upcoming Tamino Schema Editor releases, making operations like these one much more easy.