If you could provide more information about how the word sounds, maybe even a recording, it would be easier to help you figure out the correct spelling.
In the meantime, here are a few possibilities based on what you provided:
* Supercallerfragilisticexpialidoshas (This keeps the original word and adds a "s" at the end)
* Supercallerfragilisticexpialidoshus (This adds a "us" ending)
* Supercallerfragilisticexpialidocous (This changes the ending to "ous")
These are just guesses, and it's likely that the word you have in mind has a completely different spelling altogether. Please provide more context, and I'll do my best to help!