This is simplified description:

I have created a plugin and it's functions are under new menu named "bgTransform".

It opens by default with "Alt+b", event when I don't put ampersand in front of it. So these entries in source code both work:

bgTransform / f1
&bgTransform / f2

However I would like to have another menu named "fTransform" and access it with "Alt+z" ( since "Alt+f" opens file menu by default ).

Can it be done somehow ?

You can assign a key through Tools > Preferences > Key configuration. All Plug-In menu items will be listed there.

Yes, I am aware of that.
But my plugin has 23 functions as of now in three main menus and that is a lot of configuring and almost impossible to remember all.

Having remembered two or three Alt+"some letter" for opening main plugin menus would be simpler.
What I would need is something like that:

&&cMyMenu

... where "c" letter would not be visible, but would open a menu when pressed after Alt.

Thanks for answer anyway.

I'm no good at writing plug-ins, but wouldn't it be possible to write three functions which open your main plug-in menu screens and assign short-cuts to those?

Yes it would.
The problem is that I am invoking plugin functions directly on data ( like Edit->Selection->Uppercase ).
There is not much use of making GUI screen for this.
You did give me an idea though; tnx.