Approaches such as this are certainly possible (possibly with some difficulty). However, there is no guarantee that parameter values near the MAP are particularly credible. In fact, there’s no particular reason to think that the MAP itself is much more credible than other parameter values. This becomes increasingly true as the number of parameters increases. The easiest way to find credible parameter values is to sample from the posterior. Perhaps if you describe why you want the MAP + parameter nearby values, someone might suggest an easier/more appropriate approach.