is there a way to use a built-in scalar function like trim() or len()?
trinm() and len() are database functions.
Please refer to the LLBLGen Pro manual's section "Using the generated code -> Calling a database function"
As an extension to this question, is there a way to create special reusable expression classes that encapsulate such functionality (so that next time you use the same expression you can use the predicate expression class you've already built)?
You can inherit from LLBLGen Pro Predicate classes like "FieldCompareValuePredicate", and build your own Predicate class.