Can I develop constant force (pressure) from piezoelectric crystal?
Yes, you can, however there is some change (call it creep or drift or relaxation) in the motion, logarithmically decreasing with time adding up to perhaps several perecent over hours. Here is the response to a 60\$\mu\text m\$ motion: 


To get macroscopic motion from piezo elements there are a few techniques- such as stacking large number of elements, using mechanical amplification (as in benders) and "walking" or other motor techniques. The latter type of technique does not depend on the piezo to hold position so it will not have the same drift characteristic. Also it's common in high precision applications to close the loop with a feedback sensor.