The default time step in UDEC for mechanical analysis is in order of e-6. Can this mechanical time step increased to in order of 100 (e^2) like in thermal timestep where we can thermal time step to a big value.
Short answer: No.
The pdf manual (chapter 1.2.8) tells you, how the timestep is calculated. Based on this, you might be able to do some small things, e.g. increase your timestep by relaxing overly high joint stiffnesses. But other than that, the explicit nature of the code forces this timestep for stability reasons.
(The thermal calculations have a implicit mode, which is why you can use larger steps there.)