I think you didn't investigate magmi enough.
- Through datapump API, magmi can use PHP arrays as input for ingest , so you're free to fill them the way you want.
- Magmi also has SQL Datasource to have input handled by a SQL request on a database
- Of course, you could also use CSV to import
Then this is more a question of knowing how POS Winward data is accessible and then to fill up the good columns of your source in order to be compatible with magmi importable columns and also (and not the easiest part) , a way to match the product datamodel of your magento instance (since notions like product type configurable/simples/grouped and all needed attributes as well as necessary associations) may not exist in the POS Winward datamodel.
In that latter case, you need to define "semantical mapping" rules and apply them to convert one "Winward POS product" into a "Magento product"
There is no magic bullet, but magmi might be the right gun ;)