Is it possible to define an InputField in a script depending on the value of other given fields?

Yes. this is possible. Example: You want to display an additional InputField if the field "Region" has the value "SEA":

Set V[my_region] "&F[Region]"
if V[my_region=SEA]
  InputField (12,1) "Additional info for SEA countries" (12,24) Size=20 name="sea_info"
endif