English Language

[Merida]

In terms of morphology, there should be no reason why this word couldn't exist.

"-able" is a suffix added to nouns (e.g. fashionable) or verbs (e.g. edible) to form adjectives, and it adds the meaning of "in accordance to / fit to be done / subject to". ("Worth" here being a noun.)
English has the quality of being able to form compounds almost without any limits.

That being said, however, as languages are conventional and change over time, it's ultimately the usage that determines which words are "correct" and which ones aren't.

"Worthable" is a compound that doesn't go against the rules of English Grammar, but I, personally, have never heard that word.