Expressions can be created with the scripting languages supplied with the operating system by default. Typically these are Jscript / VBscript. This is different from the full featured programming languages you mention.

There is a sample script with some explanation in the PrintShop Mail installed directory, folder "examples". If you copy it to the folder "macros", you will see a new function in the list in the expression builder. (Repeatstr)
If you open the sample script in a text editor, you'll find some explanation on the matter. More information about creating them is being developed and will be published on the web site on release.

Please note that any macro created will not be supported. If you require support for PrintShop Mail, or want to report an issue, always make sure that the issue can be reproduced when no macros are in the macros folder.