Q: A word I want to use isn’t listed in the dictionary. Can I use it anyway?
A: Yes, as long as your reader will understand you.
Contrary to popular belief, dictionaries are not prescriptive manuals that tell you what words you may or may not use. They are descriptive tools that tell you what people mean when they use words, and how to spell those words. That’s why you’ll find much-reviled words like irregardless listed, albeit with a note to “use regardless instead.” You’ll also find some words, like OK, can be spelled more than one way.
Adviser is a good example. Both the -er and the -or spellings are acceptable, as far as the lexicographers are concerned. Your stylebook will tell you which to prefer. If you don’t have a stylebook — for example, in some business environments — guess what? You get to pick.
New words enter the language constantly, either by import, like lagniappe, or by coinage, like ginormous. Lexicographers collect these words like lepidopterists pinning butterflies, and when they have enough evidence, they publish their findings in updated editions of their dictionaries.
A big stink was raised among the peeververien (there’s a word not in the dictionary yet) a few years back when Merriam-Webster added ginormous to the lexicon. I’m not sure why this created such a fuss. It’s a fun word, and it was first pinned in 1948, so it’s not like they just threw it in there on a whim.
When using jargon and neologisms, just make sure the meaning is clear from the context, e.g., “posting a bunch of selfies to her Flickr account,” or provide an explanation shortly after the word is used, e.g., “You take way too many photos of yourself.”
So if you have such a word you need to include in your writing because it is the best word for the context, go ahead and use it. You do not need the lexicographers’ permission. In fact, by using it, you provide them with another sample for their tray. They will thank you for it.