It's possible but their isn't really any easy solution to it at the moment. It's more high level hacking stuff, I'll try to help you out with that if you really want it though, but I personally think it will be too much of a hassle.

it's an object that is just available by setting the 0x0E bank to bob omb battlefield
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with 0x26 commands (toads tool does this for you)
open your rom in toads tool and click on 0x26 warp destinations.
there you can set up warps.
if you set warp id 0: castle grounds->1->10
and place a pipe with bparams, so that it's got warp id:0.
if you now jumo into that pipe, it will warp you to castle grounds->area 1-> warp id 10.
place a pipe with warp id 10 or a "warp mario start" into castlegrounds or the game will crash, cause it has no destination.
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yes, use the 0x0D object bank and select bowser.
i'm not quite sure, if he will act right, guess you've got to do that with asm, i may try that later.