The way I understand it a bag of holding preserves everything you put in it and you can't put living things in there because there's no atmosphere. If you were to cast Create Atmosphere inside a bag of holding, any kind, would you then be able to put living things in it? And what would happen to everything else?
Very good and creative question. That is why I love forums like StackExchange because of the creative minds that are here and questions like yours that causes me to think as a DM and even as a player.
I know you're going off of ADnD-2e. It's been since the 80's when I played AD&D. I found my AD&D 1e and 2e DMG and player manuals.
Bag of Holding
- Adv D&D DMG pg 138 says nothing about living things within it.
- Adv D&D DMG 2e Pg 159-160 also says nothing about living creatures within it.
Regarding "Create Atmosphere" spell - I couldn't find that spell in any of my AD&D books but finally found it online (http://loemu.pegueroles.com/ADD2eWSCV1.pdf) in a "Volume One Wizard's Spell Compendium" page 188.
The Compendium states: "The create atmosphere spell can be cast on an object of any size, and will remain with it regardless of the object's gravity or state." Also, it states that it has a volume limit of "1 cu. mi. volume/level".
"1 cu. mi. volume" = "the volume of a cube with sides of 1 mile (5280 feet, 1760 yards or ~1.609 kilometres) in length" per https://en.wikipedia.org/wiki/Cubic_mile
- 1 cu. mi. volume = 147,197,952,000 cu. ft.
- Bag of holding, depending on your bag, can have a volume limit between 30 cu. ft. and 250 cu. ft.
If I were the DM running your AD&D 2e game (key word is "IF") and from my understanding of the above numbers since the AD&D books I have do not mention living creatures regarding the "Bag of Holding" at all, then I would say, why not?
However, the opening to the bag of holding is limited to about a 2' opening, based on its dimensions of 2' x 4'. So, the creature would have to fit through that opening without damaging the bag and not overload it per its weight or volume.
NOTE: If you are the DM then you can rule whether your idea will work or not. If you're a player, ask and make your point with your DM. They will make a decision based on their understanding of the rules and the dynamics of their world in which the campaign is being run in. Whatever they decide should be accepted at that point.
BUT, great creative thinking!
At the same time, there is nothing that says, a DM can't change up various aspects of magic items or creatures and so-forth to fit their campaign setting.
NOTE: I run a 5e game now. It allows living creatures to be placed in a bag of holding (PHB 5e pg 153-154) but with some restrictions on how long they can breathe in there prior to them starting to suffocate. The opening is still 2' wide. And there is no Create Atmosphere spell in 5e at this time. But, as a DM, that doesn't mean I can't allow spells from earlier systems to come into play either.