You might check and see if you can't just rebind the key in the options menu. Many games support both keyboard and mouse input, and I seem to remember that you can rebind most actions from mouse buttons to keys, although selecting dialog options (and the like) might be always mapped to the mouse.
If the game makes you use the mouse, AutoHotKey is my go-to solution for these problems. It's free, and it downloads and installs quickly, and it works with games in many cases.
In this case, you might be able to get away with:
which would map your a key to send the left mouse button, and your b key to send the right mouse button. You can replace 'a' and 'b' with whatever keys you want. Some keyboard keys have special names in AHK; there's a list of all the keys here.
Open a Notepad window, and copy and paste this line into the blank Notepad document. Save it as "mousebuttonremap.ahk" (including the quotes) and then you can double click to load it. There will be a little "A" icon in your tray that you can right click to quit the script, when you need your keys back.
Sometimes this method doesn't work with games, in which case you may want to try:
If this still doesn't work, you may have to tweak the script a bit more to work with the specific game you're trying to play. There's a FAQ entry about issues with games that might help in this case.
Update: According to other users on the AHK forums, SendEvent works best, and you may need to edit Morrowind.ini to set "Background Keyboard=1" for this to work properly.