PlaidCloud expressions and filters provide use of most non-administrative PostGIS methods. PostGIS methods are accessed by prefixing the standard method name with
ST_Expand(geometry geom, float units_to_expand);
func.ST_Expand(geometry geom, float units_to_expand)
PostGIS Official Documentation for this method can be found here.
Additional capabilities and usage examples can be found in the PostGIS documentation.