>Does anyone know how one might extend the VFP APIs? For example, the function _DBSEEK exists in the API, but OPEN DATABASE, SET DATABASE and DBUSED() and others are not in the API. I'd like to add them.

For me dropping down to C(++) makes only sense for performance bottlenecks, esp. tight loops and variable access, perhaps most strikingly in long arrays.
For all other stuff calling up into the vfp runtime should be sufficiently fast - creating tiny C(++) flls for each such case feels like make-do work.
White rooming such functionality would feel like taking advantage of the customer, unless he is warned repeatedly, including Dilbert-like formats...