How to add the white lines in the letters in Photoshop?

Ok, so it's not an exact solution, but it's definitely easier than drawing in the lines on a font that doesn't already have them.

Start with your text

Change the color of the text to white (or whatever your design's background color is) and then add an Inner Stroke of the color that you wish the font to be.

And there you have it

You can also achieve this same effect, using an Inner Glow, which will allow for more flexibility in the final result.

Use settings similar to what I have below:

And they will give you this

Mess around with it, see what you can do, or find a font that already looks the way you want it to. I prefer the Inner Glow method, because it opens up way more options (adjust your contour, choke, range, noise, etc.).