How much food can you imagine your father cooking in the 14" WSM? Maybe it holds more than you think it can...depending on whether you are grilling or smoking.There's only 5.5" between top and bottom grate, so adding a grate between those two will only accommodate thin items.
You may be able to fashion a small grate that sits on top of the top grate, as done in the larger WSMs.
^thisI don’t have a 14” but was thinking of getting one. I’m happy with my 18 and am surprised how efficient it is on charcoal.
Doesn’t the 14” have two grates? What things are you trying to cook on it? Maybe it would be better to get a cheap used 18 to supplement your 14?