Cook and hold ovens are typically variations on warming/holding drawers expanded as combination ovens. Combination ovens are variations on older pressurized steam ovens; combi ovens combine a less pressurized steam oven function with the abilities to limit the steam (steam generator) and/or humidity (humidity reservoir with agitator) and use just the heating element with a dry reservoir basin. This allows you primarily to cook well at relatively low nominal temperatures (350F and below, down to 135F) while potentially achieving a higher effective temperature with the increased humidity. The 'cooking' portion of the cook and hold is therefore a little restrained to high humidity or lower temperature style preparations–– they really also excel, however, at 'holding' foods at or just above service temperatures, whether in high humidity or low humidity (fried chicken, for instance).
Don't get me wrong, combi cook & holds are AWESOME, but you've got to know what you're getting–– which is definitely not air-flow. Rather, these guys are sometimes thought of as ideal for a sous vide-style preparation done in the oven (prime rib roasted at low temp for 8-12 hrs, then given a skin and put back in to hold for service), which is accomplished because of the high effective temperature with low nominal temperature which is accomplished by the high humidity environment; you can also poach foods immaculately well in them (lobster or scallops in butter).
My understanding is that the consensus among you smokemasters here is that low circulation is detrimental to the flavor qualities of smoke–– that a fresh, mild, continuous flow of smoke is much preferred to a stagnant contained smoke. I'm no expert, though. But it would seem to me that an alteration to the cook & hold to facilitate air-flow, by say drilling an opening for a fan, would tend to compromise its core abilities. This could probably be offset in some way, by putting the fan exterior and putting in a closure to retain more of a seal for the cook & hold aspect. But you would want to be aware of this as what I think is the essential tradeoff between an almost no (exterior) air-flow and medium-flow style of preparation.
Let me know if you guys figure something out! Alto- and Winston CVAP combi ovens are pretty expensive as-is, so I don't see a lot of future conversions happening, but would love to know if someone's got a great hack.