WIthin the game? No.
But it is possible to do that by editing the game's scripts, you can make any pokemon that has mega change form by holding a specific item just like Zacian and Zamazenta, so they'll be permanently mega pokemon for as long as they are holding that item.
If you are interested I have a quick modding tutorial for you (or for any of the many individuals that will read this in the future):
You go to Scripts folder and open PokemonMultipleForms.rb on a code editor like VS Code or notepad++, I recommend notepad++
(also, make a backup of that file in case if you mess up something)
Hit CTRL+F and search for someone that has a mega, like gardevoir, it will leave you at gardevoir's code of multiple forms
Scroll until you find weight
And insert the following code just like how I'll show in the picture below it
"getForm"=>proc{|pokemon|
next 1 if isConst?(pokemon.item,PBItems,:LIFEORB)
next 0
},
You can edit the LIFEORB with any item you want or just GARDEVOIRITE, the item in question needs to be all uppercase and without spaces, like ASSAULTVEST or something else you like to use (refer to the items.txt in the PBS folder)
And also, if the pokemon has multiple megas you'll have to change the number in "next 1" to the corresponding one of their mega, like mewtwo has two megas, X is 1 and Y is 2, or if their mega isn't numbered one in the code, but a quick glimpse to the code will tell you that.
Oh, and the picture of how the code should be, in right format:
After all of that you have to compile data, because every time you change the game's code you have to compile it, and that's only possible to do with debug, so you'll have to search for a mod that activates it, I recommend EvGym's V13 debug mode.
If you already have debug you just open the debug menu and scroll to Compile Data, hit it, wait for a while and when it's finished you close the game and open it again, it should work fine and not give you any errors in compiling if you didn't type something out of place.
edit:
Actually, in this specific case you don't need to compile data with debug, so you may skip this last part.
Just edit the script, save and play the game.
You may need to compile data for other things but not for this one,
Just like 'Haru,,' said: "Copy pasting is your friend."
Rejuv 13.5 heavily changed the script files, it's not the same file anymore but the method still works. Open Pokemon.rb in Script Folder (make a backup first), Ctrl+F Zacian: 1: Copy the 3 Zacian lines to below it; 2: Change ZACIAN to another pokemon and change RUSTEDSWORD to another item (I really recommend you to go with a Z-Crystal, Z-Moves and Megas all in one is neat) 3: If needed change the '1' to 2 or another number that corresponds to the Mega Form in question (most megas are form '1', in case of multiple megas/forms like Charizard/Mewtwo, one form is '1', the other is '2' and etc.) 4: Save, check if it works (no need to compile data or anything). the code: if pkmn.species == :ZACIAN return pkmn.item == :RUSTEDSWORD ? 1 : 0 end
