"Is there any possibility to have such buttons also in the rest of my Tk GUI?"

There certainly is. Here's part of the source code of Tk::DialogBox:

...
    if ($bl eq $default_button) {
        my $db = $bot->Frame(-relief => 'sunken', -bd => 1);
        $b->raise($db);
        $b->pack(-in => $db, -padx => '2', -pady => '2');
            ...
[download]

So, just create a Frame with a sunken border and stuff a Button with extra padding in it.

You can, of course, use that code as is; however, you may want to be a little more creative and design your own look-and-feel. See Tk::options for other ways to affect your GUI presentation. Individual widgets may have their own additional options which should be described in their documentation pages.