In the [default_gui] section of the smplayer.ini file add the buttons you want to the "actions\toolbar1=" line.
For zooming, I just remapped the zoom function to the mouse wheel, so if I position the pointer over the video, the scroll wheel zooms in and out. And if I position the pointer over the volume control then the scroll wheel adjust the volume. So I don't know what the tag names are for the zoom control.
For half speed and double speed, the correct names are halve_speed and double_speed (note the underline).
For example, I've changed my toolbar to the following:
actions\toolbar1=show_file_properties, show_playlist, show_preferences, separator, screenshot, separator, deinterlace_menu, separator, dec_speed, inc_speed, halve_speed, double_speed, separator, move_up, move_down, move_left, move_right
Going by the inc_speed and dec_speed, my guess is that the zoom tags are inc_zoom and dec_zoom.
The example above uses the icons from the default theme. if you want to use your own icons, I think you have to create a complete theme and switch to using that theme.