For now MHS isn't powerful enough for it, but it improves the efficiency to help you find what to edit.

To make it you will need to decompile the modelspc.dff from the correspounding location (take Mouth of Madness for example, find it in Manhunt\levels\pak\modelspc.dff). Here's the tool I use.Manhunt Models Compile / Decompile - Manhunt Modifications - Dixmor Hospital (dixmor-hospital.com)

Textures are also a must, you can easily edit with MagicTXD. You'll need to edit the modelspc.txd, the location is same as modelspc.dff 

Also , maybe you need to edit some ini file. But this will go depending on whether you can make hunter mod, so if you need some further instruction, I'll reply later.

With the aid of Manhunt Studio you can find the model you need to replace and the graphic you need to import. The steps are roughly as follows:

1. Decompile modelspc.dff of Innocentez levels.(mall, church2, pharm_wks) as well as Smilies levels(ansylum, cellblocks, prison)

2. Find the Innocentez hunter models(To be noted, hunter head and body are seperated) as well as Smiley.

3.Export the Innocentez textures(from Innocentez levels) and import the images into Smiley level textures.

4. As for the model replacing, the detail is a little complicated, if you need further instruction I'm ready to answer. 

Basically two ways considering models. But both needs you to unpack model files and import textures. Model file is modelspc.dff and hunter textures are in modelspc.txd

Hunter models are made up of head and body, so you'll need to find both parts for the two. 

Deliverance level's folder is Manhunt/levels/attics. Model and textures are in folder pak. You can use MMCD to pack and unpack model files. Ramirez level is called ramirez,straight.

As for the texture, you can use magictxd. Extract Ramirez's textures and import to piggsy level. Simple.

Here comes the hardest part, models.

One is straight, but it reqires 3D skills to rig and rename bones.Personally I use 3ds max. Rig piggsy with Ramirez's model, and pack the modelspc.dff

Another is subtle, you needn't to learn 3ds max, but you need to unpack MANHUNT.PAK in levels/GLOBAL/DATA. You can use MHT or Ermaccer's Manhunt PAK extracter. After unpacking, you might need to use Ermaccer's PAK Patch. There is a detailed tutorial in unpacking, so I'll talk abou replacing. Put the unpacked folder into Manhunt folder and replace everything if you need to.

Find Ramirez's models, put them into Deliverance level unpacked model folder, and then pack. Then, now that you've unpacked MANHUNT.PAK, there should be AllEntitiestype.ini in attic level. Open it, find piggsy's model name and change into Ramirez's, save and enjoy.

A music mod for the Mall Level,using Hydrogen from Hotline Miami. Put the RIB file to audio\PC\MUSIC\MALL Inspired by AsmodeuZ.

This mod adds goofy cartoon sound for Manhunt 1 executions, like bananahammer cheat of Manhunt 2. To install: Unzip and put the audio folder in Manhunt Directory, rewrite everything.

It restores beta Ramirez to Divided they Fall. To install: Put the levels folder to your Manhunt directory, rewrite everything.

A music mod for the Rabbit Level,using Le Perv from Hotline Miami 2. To install: put the RIB file to audio\PC\MUSIC\PRISON

The mod replaces gruesome execution of Blackjack and Nightstick with 0.69 one. It differs for clean game and All-weapon mod so install as your condition, don't put both! Known bug: An extra hunter grunt or weapon swing will be heard after execution.

This mod replaces the soundtrack of some typical shooting levels with Hotline Miami OST.It was inspired by AsmodeuZ To install: Put the "audio" folder to your Manhunt directory, rewrite everything. A security copy will be necessary. Known bug: It happens to custom music mod for a sudden pause. The following is the replacement list: View Of Innocent Level music:Hydrogen Completion:Turf Intro Failure:Crush Graveyard Shift Paris Kill the Rabbit: Level:Le Perv Completion and Failure:Roller Mobster Press Coverage Level:Knock Knock Completion:Inner Animal Failure:Flatline Wrong Side of the Tracks Run Trained to Kill Decade Dance Deliverance Level:Detection Completion:Dust(CB Remix) Failure:Fahkeet Time 2 Die Level and Completion:Voyager Failure:Blizzard Credit Bloodline Electric Dream