I'm looking at doing the same thing, and here are my thoughts on how to blend the two...
Do what was posted above and file off the name Greenest and carve in Phandalin. For me, this makes it an excellent tie in as the PCs have ties and connections with the town, so it makes some of the mini-encounters all that more powerful. It's a whole different deal if the hostage being threatened by Cyanwrath is Mirna who was rescued from the Redbrands, or Qelline's son Carp. The townsfolk start by trying to hunker down in the ruins of the manor. The secret tunnel is the one between Alderleaf farm and the Noxxic cavern, and you can simply relocate the other encounters to locations in Phandalin.
I would scale up the encounters by adding more creatures or replacing them with a few dragonclaws. I'd also throw in Lizardman and Bullywugs since they are so close to the Mere. You should also add a few Champion abilities to Cyanwrath and a couple other hit dice, to make it more challenging for a level 5 character.
After you get through Ep 1, I'd really suggest skipping episodes 2-4. Phandalin will be struggling after the raid, and I think there might be plenty of motivation to chase the raiders without the need to rescuing Leosin. I'm thinking of maybe even letting my group capture Snapjaw in the attack, and he might begin helping the group. However, after stabilizing the town, the cult has too much of a lead. At some point in time during the chase, I'll have the cult split up... one group going directly into the Mere, the other mixing in with caravans going south to Carnath Roadhouse. Depending on which way my group goes, they may go through the events of episode 5. If they have Snapjaw, they might be able to brave the Mere and head straight for episode 6.
The rest of the adventure plays pretty much as normal.
Since my characters have just entered episode 3 of LMoP, I'm actually going to start introducing some characters from HotDQ coming into town. At least Azbara Jos, and maybe Frulam Mondath.
Just my initial thoughts on combining the two.