Yes, it is very possible. It requires a few consoles however: x1 DS/3DS and x1 3DS equipped with Pokemon Bank. You will also need a Gen V game (I.e. Black, White, Black 2, White 2). And a Gen IV and Gen VI game, ofc. Unfortunately without all this hardware it is impossible to get your Monferno to Gen VI. If you fulfil the requirements, great! Keep reading.
Insert your Diamond cartridge into the DS/3DS and insert the Gen V game into the 3DS equipped with Pokemon Bank. In your Diamond game, make sure Monferno is in the PC and not in the Daycare or whatever. Initiate your Gen V game and head to the PokeTransfer Lab. Talk to the dude there and your game will start searching for nearby Gen IV games. On your DS/3DS, initiate DS Download Play and find the Gen V title. Follow the prompts shown on both devices and one minigame later Monferno should be in your Gen V game.
Now start up Pokemon Bank with your Gen V game still inserted in the console. Follow the prompts there in order to transfer Monferno into the Pokemon Bank. Reopen Pokemon Bank with your Gen VI game inserted in the 3DS. Follow the prompts and you should have Monferno in your Gen VI game!! Have fun.
If you want me to clarify anything, feel free to ask since this explanation seems too brief lol.
Hope I helped. :)