Well, you can set up a voltage source to look like a serial bit stream by linking it to a PWL file, but that'd be pretty cumbersome to generate for more than a few bytes.
There may be third party models that you could find or create that could be used to extend the logic simulation capabilities of ltspice.
Understand though, ltspice is an Analog simulator. There are some built in logic functions, but those are there to support analog operations.
If you want to do full logic simulation, including serial protocols, I think you'd be better off using a logic simulator.