Endgame FEN generator is it possible to generate FEN strings for endgames based on specific criteria:  n-pieces, w/d/l, and side to move?

So say I wanted all FEN strings for King plus Rook vs King plus Pawn (KPKR) endgames, where it is white to move, and white draws with perfect play.  

I know there would be an extremly large number of results, and the next step would be to then categorize those results further by other criteria, like say white king is in front of the pawn, or the black pawn has passed the 6th rank.

I assume an endgame tablebase (egtb) has this information stored in it already, but how to get this information out of them?  Or is there some other way of generating this?

Thanks!!
Firstly, FEN notation is used to describe a board state or permutation, not a family of board states or permutations.

What you are talking about, is a way to index board positions based on your criteria. i.e. Rook vs Knight end games or such

This can be done by most database software (Fritz, Chessbase). This is done by searching games in the database, and pulling them out based on the board positions. 

For example, if I want to get a bunch of rook vs knight end games, I can search for all FEN strings that contain only K,k,p,P,r,R,n,N. Basically, positions that contain pawns, knights, rooks and kings. 

As for egtb, the format are stored using bit bases and the positions are assigned to a unique index. I would assume this is how Chess Position Trainer stores its FEN strings in order to determine transpositions.