Generally speaking, you need some sort of narrative device to make the split make any sense. Basically, what Ral said. If you establish a pattern of having some chapters be 1st person from the main POV narrator and other chapters be 3rd person from the POV of other characters, that might be one way to do it.
I once wrote a story in the 1st person, but the narrator was also relaying stories she learned from other characters through interviews and conversations, which she then related in the 3rd person. If something like this is happening, you just include it in the main text (or maybe make it a separate section). For instance,
When I was done with my daily rounds, I asked Hal what in the hell had happened to him the other week that had landed him in the hospital...
Last week, when he was still on second shift, Hal got a message from Dolores (yes, that Dolores) from the investigation group. She needed to speak with Hal ASAP in order to sort some insurance things so Hal could pick up his sister's settlement, so of course he scrambled at the opportunity.
"Listen," Dolores said, "it's going to have to wait until Monday, unless you can make it in the next twelve minutes."
Hal figured he was about ten minutes away, so he jumped into his Jeep and off he went.
So here I've just transitioned from 1st to 3rd person. Technically, I'm still in 1st, but since our narrator is removing herself from the story (since it's being told to her), there's no 1st person unless she wants to chime in to editorialize. That's one way to do it.
