Lectora has MouseEnter and MouseExit triggers but does not support focus and blur triggers. However, you can write a page level action that will set focus and blur events of the HTML elements and map them to the MouseEnter and MouseExit actions that you already have. With this, when you tab to the button, focus event will fire and then it will call the MouseEnter action function that you already setup.
Have a look at the sample title and try it out. Especially look at the page action (OnShowRunJS) which does the mapping of focus events to MouseEnter action functions. This approach only works after publishing or when you do Preview in browser. It does not work in Run Mode.
Also, note that button40, button63, etc. in the page action refer to the HTML names of buttons. You can get those by selecting the button, going to Properties tab and hover or click on the small expand arrow in the lower-right corner of the first group of controls.
Hope this fits your needs.