Technically, you can already do this, albeit only with unhatched Pixinytes.

i think i might have to try that actualy and hope it still works >:)c

(also like ik this might come in a later up date BuT ThEY aRE So0oO smOL i JUsT waNNa HhhHHHHh, but like no rush on the devs its chill :D)

To add a bit to what Cherche said: even though there are only 21 pixpals at the moment, each one has 3 variations, so altogether the current pixpals consist of 63 images.

If we think about the overworld (house) sprites, each pixpet consists of 8 images (2 for each direction).

Knowing that, hopefully it becomes a bit easier to understand why overworld sprites for 63 pixpals would take quite a long time without a bigger art team. D:

But I do really love this idea and want it to be a reality someday. Itty bitty pals in the house would be super cute