GIMS IS VERY EASY PLUG-IN, if you learned 3dsmax i bet you'll learn GIMS.
GIMS tool is used to export import openformats from OPEN IV to GIMS and vice versa.
GIMS have a few helpers like a

map reference of the real one in gta iv,

material editor(to create and edit the GTAMATERIALS (shaders and collision types),

LOD manager (to manage the lods of the map mods(if exists)will search for objects with LOD_ prefix, gta configurator(to handle the gta.dat and images.txt files and manage them)and more things but a real import one is the

HIERARCHY MANAGER that allows you to set properties to the objects(model,collision,node and dictionary mode(odr-odd,obn-obd)and also

IDE/WPL HIERARCHY to set normal, stream, strbig WPLs and IDE files.
there is also

IDE EDITOR for ide properties of objects or instances(just like draw dist,texture assignment,flags...)

IDE 2FX for objects with an fx function like scripts (amb_savebed,amb_tv....)and more issues

WPL EDITOR.to set wpl placement parametres(at the mapping time)

NOD PATHS EDITOR some serious sh*t comapared with the previous ones(this one is to create paths on game for cars,peds and all that stuff)
-other sections are the same, just tools to create-edit GTA IV stuff.
TO REPLACE PEDS PARTS:
1.look for a ped to export(you need OPEN IV to export and open format (odr instead of wdr,supported in GIMS)
2.import the whole odd to the 3dsmax scene by the import tab in GIMS
3.edit your part of the body or whatever but
THIS IS IMPORTANT!!!copy the object you'll replace in the scene to have the skin modifier (you'll need it later)(the modifier i mean)
4.after editing (editable mesh or editable poly)just copy the skin modifier of the old object and paste it to the new one and export the whole odd.
5.CAUTION!!!after doing anything(editing,exporting)make sure that the "always arrange" mode is enabled in the schematical view because the model dictionaries have Hierarchy (model PED ID:, in the pedVariations.dat) important.

i was incredible bored to write this text so i hope this info was helpful.

NOTE: other plugins are not relevant for 3dsmax because GIMS can make almost all of it.
This post has been edited by aacclapd on Friday, Jun 29 2012, 22:45