I have no prior experience with this but there seem to be tools out there that would help; one example is here
Does MacPosX have the ability to export to different formats? It may be that it can export tables to a format that MySql can import.
Hopefully a seasoned (Mac) developer here will have some more specific advice!
The extended features of the POS software would not be transportable though, i.e. the invoice printing and scheduling or the label-printing and barcode support; but if the inventory and customer details and sales details tables are exportable, then the data from these may be importable into mySql (perhaps as csv files) and could be used for the functionality that Powerstore offers.