It's been years since someone has done this for Blender so now that I know enough about how it works I made a basic tutorial in video and text. Kitbashing is actually easy to learn once you get over the initial creativity curve. WillJay84 made a great tutorial on animation so instead I will eventually make more tutorials and videos for Blender.
You can use CASCView to extract files but it's not required since the SC2Editor has its own export function through the native file browser. Most importantly though, you need Blender and the Talv's m3addon. Remember to save often, especially before exporting anything.
1. Open Blender. Go to Edit > Preferences > Install the addon from file.
2. Restarting Blender shouldn't be necessary but won't hurt. After this, import the model that you want to take parts from.
3. Delete its armature. Often times meshes or body parts such as armor, carapace, weapons, etc are separate objects once you import.
3a. If your desired part is merged with something you don't want: Select the object > Edit Mode > Select Faces > hover over desired part > Press L (Select Linked Objects). If you're familiar with 3DSMax element select this is basically the same. In some cases you will need to manually select faces because pressing L selects too much.
4. Press P to separate any Linked Object selections you make to a different mesh object.
5. Delete what you don't want, and go into the modifiers for each unanimated object. Delete the UseArmature modifierin red highlight.
6. Export the "parts" .blend file as its own .m3. Then make a new project and import the main object that you will be modifying.
6a. For example, you want to give the Purifier Adept the same chest as the Sentinel Zealot. Edit the Sentinel Zealot first, subtracting everything but the chest armor, then export it under your name (I'll pick ZealotChest.m3). Make a new blend file, import the Purifier Adept, then import the ZealotChest.m3 and it will be right on top of the Adept.
7. Click on the Purifier Adept's armature, click on the Object Data Properties tab (green stickman) on the control panel to the right. Now click on Rest Position to put the mesh in a neutral state. Don't try to finagle objects into position and parent them to the armature while it is in an in-game pose - you make your life harder because mesh weighting problems will occur in force!
8. Position the chest armor where you want.
9. Select the Zealot chestpiece and ctrl+click on the Armature for the Adept. Ctrl+P and select Automatic Weights. Save yourself headache and do this individually for each separate StaticMesh that gets imported.
10. Weight paint as necessary to prevent unwanted deformation of meshes.
11. Go into the Playback timeline - click on the clock, change it to Dope Sheet, then directly on the right select Action Editor from the dropdown and then select the Armature. You can inspect the animation integrity before you make your final export.
12. Remember to save! Import any custom textures into a test map, save the map, then import your custom unit into that document.
13. To make sure the export worked correctly, preview the model in the Cutscene Editor from the Import Module and comb through its animations for irregularities.