Blender addon to import .obj files containing morph targets, based on https://gist.github.com/Marcus-Zhu/6e397f6b5d295ab87602a26aef0673a0

ObjSeqImport.py

bl_info = {
    "name": "Import OBJ with morph targets",
    "author": "Trass3r",
    "version": (1, 0, 0),
    "blender": (3, 2, 0),
    "location": "File > Import/Export",
    "description": "Import OBJ with morph targets",
    "doc_url": "",
    "support": "COMMUNITY",
    "category": "Import-Export",
}

import bpy
from pathlib import Path
from bpy.props import (
    BoolProperty,
    CollectionProperty,
    StringProperty,
)
from bpy_extras.io_utils import ImportHelper


class ImportObjSeq(bpy.types.Operator, ImportHelper):

    bl_idname = "import_scene.objmeshseq"
    bl_label = "Import OBJ with morph targets"
    bl_options = {"REGISTER", "UNDO"}

    filename_ext = ".obj"
    filter_glob: StringProperty(default="*.obj", options={"HIDDEN"})

    useRelativeShapekeys: BoolProperty(
        name="Import as relative Morph Targets",
        description="Import frames as relative or absolute shapekeys.",
        default=True,
    )

    def execute(self, context):
        self.create_shapekeys(self.filepath)
        return {"FINISHED"}

    def create_shapekeys(self, filepath):

        bpy.ops.import_scene.obj(filepath=filepath, use_split_objects=True, use_split_groups=False, use_groups_as_vgroups=False)
        # all imported objects will be selected

        baseObject = bpy.context.selected_objects[0]
        bpy.context.view_layer.objects.active = baseObject

        bpy.ops.object.editmode_toggle()
        bpy.ops.mesh.remove_doubles(threshold=1e-05)
        bpy.ops.object.editmode_toggle()

        bpy.ops.object.join_shapes()
        # remove all but the base object
        bpy.context.active_object.select_set(False)
        bpy.ops.object.delete(use_global=True, confirm=False)

        baseObject.active_shape_key_index = 0
        for face in baseObject.data.polygons:
            face.use_smooth = True
        main_key = baseObject.data.shape_keys

        if self.useRelativeShapekeys:
            # set keyframes
            main_key.use_relative = True
            for i, key_block in enumerate(main_key.key_blocks[1:]):
                key_block.value = 0.0
                key_block.keyframe_insert("value", frame=i)
                key_block.value = 1.0
                key_block.keyframe_insert("value", frame=i+1)
                key_block.value = 0.0
                key_block.keyframe_insert("value", frame=i+2)
        else:
            # absolute shape keys
            main_key.use_relative = False
            bpy.ops.object.shape_key_retime()
            fcurve = main_key.driver_add("eval_time")
            fcurve.driver.expression = "frame*10"
            assert(fcurve.driver.is_simple_expression)

        # set start/end time
        bpy.context.scene.frame_start = 0
        bpy.context.scene.frame_end = len(main_key.key_blocks) - 1


def menu_func_import(self, context):
    self.layout.operator(ImportObjSeq.bl_idname, text="OBJ with morph targets (.obj)")

def register():
    bpy.utils.register_class(ImportObjSeq)
    bpy.types.TOPBAR_MT_file_import.append(menu_func_import)

def unregister():
    bpy.utils.unregister_class(ImportObjSeq)
    bpy.types.TOPBAR_MT_file_import.remove(menu_func_import)


if __name__ == "__main__":
    register()