Hello! May i know, it is allowed to use transparency colorkey on spritesheet gfx? Can't clarify it for myself after reading terms of jam. I am using different engine, not PICO-8.

The basic sprite drawing function in PICO-8 uses a list of transparent colors in the draw state, which is basically the same thing. Also I'm not entirely sure how you would use a spritesheet otherwise. 

Thanks. There is no other way, just can't figure out this

Yes, you can make any changes to the spritesheet. :) 

If you want to try to follow the restrants of the jam you can uniformly edit a sprite as in any of the ways you can do it with pal statements or I think the 90˚ flipping or 180˚ mirroring is fine, normally in Pico-8 black by default is transparent so if you want to just use them the way they are you can set that to alpha if you want (though of course if you want to go nuts and not worry about it feel free.)

Yeah, you can make colors transparent, rotate, flip -- just not ADD information/detail to sprites.  Many of them are made to use palette and transparency tricks.