I'm not aware of any official API but more than a decade ago back when I was working with Big 4, they did have dedicated teams that built bridges to export data to proforma their own proprietary electronic organizers and to import data that were mapped line-by-line to each input screens, both for GoSystem RS and ProSystem Fx.  That technology actually improved and evolved over a good number of years.

The built-in functionalities in tax software seem to be very limited, mostly for FAM and Trial Balance.  You'd probably need quite a bit of resources and proprietary knowledge of the database structure, etc., to do anything more and to keep up with their development each year.