Hive rules "Please do not double post in threads or make two of the same thread. We have an edit button for a reason" as well as "To bump a thread on an Internet forum is to post a reply to it purely in order to raise the thread's profile. This will typically return it to the top of the list of active threads...Threads bumps must include new details and / or information, not just simply a word or vague comment...Threads cannot be bumped within 48 hours of the most recent post"
with this said,
To equip custom attachments ingame, you create an ability/buff to give to a unit, then give him an "attachment" (there is a field for this in the object editor), then you make sure that the "required attachment names" (another field) are correct, e.g. "LEFT HAND" if the attachment is a sword etc.
attachment points include, "head, arm, hand, leg, chest, weapon, foot, sprite (large,medium,small)" that you can pair with words like LEFT and RIGHT
(note, not all models have all of these attachment points, and some custom models may have oddly names ones)
to put a custom attachment in the MODEL itself, thus making it a custom model, you need to open the attachment in magos, export the geoset, import the geoset in the other model onto the correct BONE, apply the appropriate material/texture. assuming that you have the author's permission for both attachment and unit model.