Remap the . in the numpad?
This thread on superuser discusses a similar problem. The accepted answer points to this key mapping program which you may find meets your needs.

The other, more robust but slightly more complex solution, would be to create your own custom keyboard layout using Ukelele. You can then install your custom layout in your  ~/Library/Keyboard Layouts folder, and activate it using the System Preferences -> International pane. (the best way to use Ukelele is to duplicate your existing Turkish keyboard layout file and modify just the single key in question). 

As a fast solution, and if you don't mind changing other keys on the keyboard, you could use the International pane to switch to a US keyboard layout.