Unfortunately, importing sprites doesn't adjust the palette accordingly so that's out.
I think you'd need to poke the required RGB values into TICs ram. The only way I can think of getting those values is to write an external program that will allow you to load an image, read RGB values then write them to a text file (there are dozens of free languages/libraries that can do this - for example LOVE2D if you want to stick to Lua ). You could then paste the values into your Lua file and poke them into RAM.
Maybe there's a simpler way but I can't think of one!