we are trying to create a map/reduce script to to update all that shipment date via an api , i know that the inbound shipments records are diffrent would this be do able ?

Did you try first? What problem are you running into?

i have not started we are in planning stage the question is if we should due it via map/reduce script or thru API calls in our outside application

I give my vote to a MapReduce script... Everything possible through API will be possible with SuiteScript in M/R