convert_free.py

import bpy
from mathutils import Vector


def convert_curve_to_gp(obj):

    pencil = bpy.data.grease_pencil.new("pencil")
    scene = bpy.context.scene
    scene.grease_pencil = pencil
    mesh = obj.data
    
    layer = pencil.layers.new("layer", set_active=True)
    frame = layer.frames.new(scene.frame_current)

    mesh = obj.to_mesh(scene, apply_modifiers=False, settings='PREVIEW')
    verts = mesh.vertices
    for edge in mesh.edges:
        stroke = frame.strokes.new()
        stroke.draw_mode = '3DSPACE'
        stroke.points.add(2)
        stroke.points[0].co = obj.matrix_world * verts[edge.vertices[0]].co
        stroke.points[1].co = obj.matrix_world * verts[edge.vertices[1]].co
    
    bpy.data.meshes.remove(mesh)

obj = bpy.context.object
convert_curve_to_gp(obj)