Adding -able to transitive verbs is still a fairly productive process in today's English.
Since toggle is a transitive verb, you should be able to form toggleable (meaning "able to be toggled") and be understood, even if the reader has never seen the word before. There are lexical exceptions which -able doesn't usually attach to (beware, want, loathe, etc.) but I don't believe toggle is one of them.
When you add -able, the syllablic /əl/ in toggle is likely to become a regular /l/, becoming the onset of the following syllable. This is called syncope and is sometimes reflected in spelling--particularly in derivations like these as there's no history to force the spelling one way or the other--so you might instead spell it togglable.
Of course, as jmadsen points out, there's no reason you have to add -able. But you can if you like.