No. That means something completely different. They're saying (probably hyperbolically) that they cannot imagine the first of two possible situations, not that they can imagine the second.
It is in fact not the case that you can't say that. Double negatives, like sentences with a excessive number of conjuctions, are perfectly grammatical (when they don't violate some actual grammatical rule like the one "You do can say that." does); rather, they are in most cases stylistically bad, for much the same reasons that:
Oh. ... Okay, then. In that case I regret to inform you that your previous comment was extremely poorly phrased, to the point of seeming to mean almost the opposite of what you apparently intended. (Particularly in that double negatives are fine in conversation or writing, and can in fact be used, wherever they actually make sense - which is relatively rare in general, but occurs in the case we were originally discussing, since "can imagine it would be" means something different.)
> I can imagine it would be
No. That means something completely different. They're saying (probably hyperbolically) that they cannot imagine the first of two possible situations, not that they can imagine the second.