To be honest, I still don't quite understand you because you seem to confuse some terms. However, I think you want to use some sort of scripting or programming language as extension for your Lite-C application which is absolutely possible and legal. There are several approaches which you can use. The easiest way would probably be by including an existing library such as LUA, Python, etc.

If you really want to design your own language, you will have to write an interpreter to parse the code and something to execute the parsed instructions, most likely a virtual machine. Depending on how complex your needs are, this can be quite easy but it can also be quite hard.

Again, sorry if that's not what you want to do.