Yeah, you already can:
Custom borders need to be in `8bpp bmp` format, renamed as "border.bmp" inside the `_gba/` folder in the root of your SD card.
You can also add per-game borders, they just need to be renamed as the game internal ID + rev number, ie. BPEE00.bmp, and placed inside `_gba/borders`
You can change the borders behavior in the settings file, the "borderimage" key can be set to either "default", "game" or "none":
JSON:
{
"runSettings": {
"enableWramICache": true,
"enableEwramDCache": true
},
"displaySettings": {
"gbaScreen": "top",
"gbaColorCorrection": "Agb001",
"gbaScreenBrightness": 16,
"enableCenterAndMask": true,
"centerOffsetX": 8,
"centerOffsetY": 16,
"maskWidth": 240,
"maskHeight": 160,
"borderImage": "default"
}
}