Disastrous has three syllables but, yes, an extra syllable is often added in speech due to the relationship between this word and the word disaster.
Neither it is an added syllable
A prefix is a syllable that is added to the beginning of a word to change its meaning or create a new word.
prefix
prefix
prefix
The syllable added to the end of a word is called a suffix. It can modify the meaning or change the grammatical function of the word.
A "prefix". However, note that some prefixes, such as "ante" and "anti", have more than one syllable.
prefix
Suffixes are things added to the ends of words, although they can be more than one syllable long.
A suffix.
A suffix.
I believe you're referring to a prefix.