Would a MTB frame be too long if fitted with drop handlebars to build an adventure bike?
How long is a piece of string?

Any bicycle is possible, which is the beauty of a bicycle in general. Bicycles can be anything really - roadies, MTB, single-speed, recumbents, even giant A-frame types that have steps to get to the saddle!
Your question is very theoretical and will require a proper re-write because you're not being specific to a particular bicycle or size that we could say ok.. here's what you need.

As an "answer", all I can say now is that you take your frame and your new build to a competent bike fitter and get fitted for the bike! You may find all that geometry might have to be thrown out the window for the sake of your posture, back and skeletal health.

Good luck!