Hello, Daniel.
Yes, it's possible!
With the help of Rockchip SDK and one of those C compilers (such as Visual Studio)
you will be able to create the games, apps and even firmwares for your player.
You should try to find the Rockchip RK26xx CPU based player.
(However, it's still impossible to get the firmware backup from these players;
but the most of RK27xx-based players don't support the .rkp games)
It is not an easy task: there's no doubt that
only a few sellers know much about their players
and can provide the information about player's CPU.
Maybe you should search in Google Pictures for "RK26xx"
to find the images of the RK26xx-based players,
and then check if the seller's player have the similar main menu.
But the better way is to ask the seller to remove the battery from the player
and send you the information that was printed on the chip -
it will tell you if it's the RK26xx player;
and check in the description from the seller's site (or ask the seller)
if this player has the Game Menu:
maybe there're some players with the right CPU
but their firmware don't support the games.
P.S. xx is two numbers. The best chip from RK26xx series is RK2608.
(it has the same features as RK2606 + FM Radio)
I don't think that such an old chips as RK2602 will support all the available games.
Here are the photos of my player:
Soundbreeze Mp-401 - MyMPx - Chess game
Album - Main menu
P.P.S. The Rockchip-based players have the 200 MHz CPU.
There's even a 3D Game (called Wolf 3D) that was ported to the Rockchip platform.
Wolfenstein 3D - Wikipedia, the free encyclopedia
Rockchip Game demo Wolf3D! - MyMPx
If you have any other questions, ask me!
Hope your search will be successful.
Bye!