Sure you can! Choosing to tell your story in first person doesn’t mean forgoing subplots that don’t include your narrator. Just be sure that your narrator can know and thus mention enough of the subplot’s events for readers to follow that storyline through the book. Or, if you want your narrator to be oblivious to the subplot for most of the book, have him observe or be involved with behavior and dialogue of other characters that somehow reveals subplot clues to readers. This’ll prime readers for the subplot’s eventual full revelation.