Create your own customized antenna by importing a planar mesh. For more complex 2-D structures, use 2-D shapes provided in the Antenna Toolbox and perform Boolean operations on them. The resulting shape can be a 2-D antenna, or it can be describe a specific layer in the PCB stack.
To author 3-D antenna geometry, Use Stereolithography (STL) files. Define the feed for this arbitrary element and analyze the antenna or array using existing analysis functions.