the throttle body i used at my 91 Caprice for the TPI swap is in veeery used condition, so the "shaft" of the throttle blades moves forwards and backwards about 1 mm

The air, which can flow around the blades in closed throttle is not the problem, but with that i can't make a correct adjustment of the TPS sensor.

So the question is, is there any way to fix this? or will i need a new throttle body?

 

The throttle body can be drilled, bushed, and then reamed to the correct size. Chances are the throttle body itself is worn much more than the throttle shaft, but check the shaft diameter once it is disassembled to make sure it isn't worn beyond usefulness.

If you were located in the States, it would be easier/faster to find a good used TB in a salvage yard, but availability there may not be as broad.