Being an assembly capable hacker would all but guarantee this being possible. Without it... you possibly have some options.
1) Character selection cheats. Normally seen in a fighting game or racing game but generally people making games will not code something special for boss/hidden characters and just deny you a chance to select them at the menu and thus use them in the game. As your selection is just a number then if you can force this number (like with a cheat) to be something else then you get said hidden character.
I can see it going both ways here and said house bots being effective unselectable characters, and I can see them being NPCs with their own code just as happily.
If you wanted to play final fantasy overworld as a NPC then find the graphical tile data and do some copy-paste action (maybe also sorting any palette issues). Battles for a 2d game are harder as it is unlikely the devs coded full battle animations for npcs. If you want to have some fun with a tile editor and play pixel artist though...
In this case however the house bots likely will have all the necessary angles and attack animations if they operate within the same plane.
Possible issue. I never saw the version of robot wars this was based on but the UK one did have rather larger bots than competitors were allowed, if this is the case here then the sprites might be bigger than normal player bots and thus not work, or take a bit of fiddling to fit.
Possible second issue deals with hitboxes, especially if they are bigger, but cross that bridge as and when.
For 3d games then if the NPCs are similar to the PCs (and if they are all essentially the same character that is a distinct possibility) you can usually twist the 3d animations to your ends (or indeed just overwrite the model and texture and leave 3d animations alone). Looking at a video of this then the jerky turning animations would have me believe it is actually an isometric game rather than proper 3d (the GBA did have some games doing this) and thus you are back at sprites.
3) Functional replacement.
Again more for 3d or for something that might be a hidden extra mode where you can play as a boss or something. Easier if there is a file system involved (not likely on the GBA, is on the DS, hence all sorts of mods like this there for various RPGS and whatever else) but much like character selection here you find all the data controlling the character and either point the game at it or overwrite the original. No need to understand anything, just need to know where all the relevant parts are.
With the possible exception of the cheat thing and some of the things with file systems then none of those are necessarily trivial if you are coming in completely cold to hacking. At the same time while all will be able to benefit from a knowledge of assembly hacking there are countless examples of it being done by people with fairly minimal skills.
If you did have some assembly skills then as you are effectively doing things the game expects to see* then it is not so bad as completely altering behaviours often is where you are compelled to code your own stuff.
*this said one of the most well known examples of this would be Crash Team Racing where you play as a boss character. Apparently it causes the system to run out of memory on some tracks and thus crash the game.https://www.youtube.com/watch?v=-r-oaaWsPE8