Is there a way to use abilities at my mouse cursor when I press the hotbar button?

There is no way to do this in-game. You'll have to use a third-party program (AutoHotkey, AutoIT) to do that.

Here's an AutoHotkey script, taken from the official Steam forums:

#NoEnv  ; Recommended for performance and compatibility with future AutoHotkey releases.
; #Warn  ; Enable warnings to assist with detecting common errors.
SendMode Input  ; Recommended for new scripts due to its superior speed and reliability.
SetWorkingDir %A_ScriptDir%  ; Ensures a consistent starting directory.

#SingleInstance
#InstallKeybdHook
#InstallMouseHook

SetTimer, MoveTimer, 128

return

MoveTimer:
IfWinActive, Titan Quest
  {
      GetKeyState, state, MButton ; or what button you want it to be
      if state = D
      {
         MouseClick, left
      }
      GetKeyState, state, numpad0 ; or what button you want it to be
      if state = D
      {
    ;MouseClick, left
      }
  }
return