Yes, 'drawable' is a correct and legitimate word
I don't understand why you can't use drawable to mean 'something that can be drawn'. Perhaps it's just its unlookupability (or unlookupable-ness) or maddey-uppy-ness, or its freakiness or unprecedentedness or even its unapproachableness, but don't let those factors stop you from using 'drawable'.
Dictionaries don't make words; people do! It doesn't have to be in dictionaries for it to be a correct and legitimate word. You can make up any word so long as it's understandable. Also as tchrist said in one of his answers: 'No dictionary can ever tell you something is not a word!'.
It's Modern English and you can adjectivise just about any verb because most verbs in Modern English are adjectivisable. The suffix -able is a highly productive suffix in Modern English and is appendable to an enormous number of words (usually verbs).
In his English suffixes: Stress-assignment properties, productivity, selection and combinatorial processes, Ives Trevian says that -able 'has yielded a considerable number of adjectives endowed with the
senses “capable of, deserving of, likely to or tending to"'. He goes on to say that able-ise-able1 verbs are chiefly transitive. Note, however, that it is appended to the infinitive form, so it should be draw + -able, not *drawn + -able. Also see envisageable, which is not present in any dictionary, but is still correct and understandable.
Moreover, it can have different meanings, depending on the context as FumbleFingers pointed out in a comment beneath the question: Besides drawable = capable of being sketched / represented pictorially, there's the sense involved in drawable funds - money held in a bank account or similar, which can be [with]drawn and spent. Not to mention drawable wire, with various senses centring on draw = pull [tight], as in drawstrings.
- Able-ise-able isn't present in any dictionary, I just made it up to demonstrate that you can able-ise (or ableify) a large number of verbs. Also note that ableisation only works well with verbs. -ise is a verbifying suffix and the suffix -able is immediately attachable to it:
- able + ise -> able-ise
- able-ise + able -> able-ise-able