Not exactly what you want but you can use buttons on hotbars you don't see if you use the right keybind. Bar 1,2,3 is visible for you and if you want e.g. use a skill button which is on the first place on bar 4 (which is not visible) by pressing e.g. the F9 button then the bind must look like this:
/bind F9 "+TrayExecByTray 3 0"
Note: The internal number for the first hotbar is 0 and the internal number for the first button is 0
Therefore the two numbers at the end of the bind command (3 0) are meaning hotbar 4 button 1
P.S: Even if you don't want use such keybindings you should have enough place on 3 bars because you don't must see the 5 shield buttons (keybindings for them are in the option menue). The transwarp button and the abandon ship button are rarely used so it can be moved to e.g. bar 4. In case you use a autofire macro most of your weapon buttons can also be moved to bar 4.