It is only possible to increase the length a city can hold out by either Upgrading the settlement to maximum or, i believe, having it max garrisoned also helps But no there isnt a way to modify the files to have a "Set" amount of turns, as others have said it is hard coded.
As a side note.... I personally think it would take the fun out of it, I like the race to reinforce an undermanned settlement.... but thats just me