Can I create my own symbols?
Yes, you can create your own symbols.
How do I create my own symbol?
Start with the menu command File=>New Symbol.
Can I create my own switching regulator models?
Not very easily. The switching regulator models that ship with LTspice XVII use a new hardware description language and new intrinsic SPICE devices designed to encapsulate the behavior of LTC's switching regulator products. Even if you succeed in making a model with standard SPICE primitives, the simulation will run orders of magnitude slower. Note that some people have made such switching regulator models with standard SPICE devices. LTspice can run these models and will usually outperform the simulator for which they were targeted.