VS 2010 Add value to Excel value; is it possible?

I do not know if this is even possible via VB, but what I want to do is add a number from a textbox (custom named "hours") to the same cell it is being placed in to. So, if there is already a number in the cell, it will automatically add it instead of just erasing any existing value and no addition taking place.

Here's the line of code I attempted this with (does not work, though)
Suggestions/feedback on if or how this could be done?