Okay you will need Frosty's cache editor
And you will also need RSDataSuite.
If you have them both, cool, will save you some time to google them and download them from various R-S threads.
Focussing on your cache that you want to EXTRACT from:
You must find the desired item model numbers from an item, or an npc.
So open frosty's cache editor, find your cache that you wish to take models OUT of, go to Items/NPCs and look for your item/npc,
Hit edit, Model tab, you will find the model numbers there.
Now, write those numbers down somewhere on a blank notepad.
Time to open up RSDatasuite, open the cache you wish to extract from again.
Click on Models once it is highlighted when you picked your cache.
Scroll down and look for the numbers you perhaps wrote down on your blank notepad document (or the ones you memorised)
Save the models somewhere you will remember to locate them.
This is where we will focus on the cache you will PACK stuff to:
Then close frosty's cache editor and re open it, load the cache you wish to PACK to (new cache)
Then go items/npc's whichever models you wished to add
Edit a null or an existing item/npc, 'add models' .. when it asks to specify a new model number, say no..
Find the models you have saved from the old cache when you used rsdatasuite, pack them... the NEW model ID numbers will appear in the Frosty cache editor log panel
Enter the NEW assigned model number into the model section of the item/npc..
If you have any more questions or still need help, just pm me