I think you can. Heck, even a chapter can be made to be somewhat standalone with a problem, resolution, reaction, and a new problem at the end, repeat endlessly. Chapters have problems, groups of chapters have bigger problems, novel have bigger problems and themes still. The reader needs an incentive to continue from start to end. Trilogies are big works that absolutely need this since I don't think any reader is going to read 400K words w/o some thrills, rewards, and satisfaction alone the way. So, all you need to do is end the first novel with some type of a resolution strong enough to satisfy the reader and give them a warm feeling.The whole premise of the story is about a magical event that takes place at the end of the trilogy. I can't write it into the first book and make it a stand alone with more to follow.
On a personal note, as a kid I absolutely hated books that ended on cliffhangers. It was like the author lied to me and sold an incomplete product. If I'd known it was a cliffhanger, I wouldn't have brought the novel, only to wait a year to find out what happened. That's assuming the following novel ever gets published.
That's doesn't mean you can't end the novel with trouble on the horizon, but there should be a sense of completeness. If you're telling the agent you can't do this, they might see it as a sign you haven't mastered the art of narrative structure.