Implement custom (external) commands which can be accessible from menu or tool bar

Already exists in the preferences, Tools page.

I mean execution of a command itself, not a showing me a terminal window. Like a calling external tools in qt creator.

What exactly do you mean then with "execution of a command itself"? What command you like to invoke?

for example

find . -iname "*.qmltypes" -exec sed -i 's/.*DLT.*WARNING.*/ /' {} +

or git clean -d -f- x

or something like make -C ../build/ generate_assets

I would like to have possibility to execute user defined command by pressing a button or a menu item

You may put this command sequence into a shell script and add an SmartGit tool to this shell script.