the easy way, with no painstaking manual copy-pasting in LNZ pro!!
this was written for clothez files, but i'd think it works similarly for toyz.
instead what you'll need is resource hacker, which works pretty similarly to LNZ pro as it's also a... well, resource hacking program, but it has some nifty extra things that make it way more useful for this specific task.
alright so this is by far the easiest and most effortless method, i think? all you need is an item that is based on a clothing item that is already present in 3,
or still present in 4 - items like the pink bow, plaid sweater, any of the socks, etc etc.
copy over your .clo file from one versions' clothes folder to the other. i personally have only done from 4 to 3, but this should work the other way around too.
now open resource hacker, and open the clothing item that yours is based off of in the target versions' folder. for example, if your items based off of the p4 pink bow, you'll open the p3 pink bow.
now at the top bar, go into 'action' and pick 'add from a resource file' - let it load all files to see .clo files and find the item you copied over.
the import resources window should pop up. select 'overwrite' duplicates, check all, and import.
now 'save as', and overwrite the other version item you copied over, keeping the exact same name.
that's it! if you load up your game, it should be in.
because both items had the same sprite name, it just automatically pastes over all of your modified data including stuff like filmstrips and new ID numbers onto the new one.
this requires a bit of extra effort and a hex editor of your choice as petz 5 handles filmstrips in a completely different format.
but the first step is similar to the previous method. copy over your custom 4/3 .clo file to your p5 clothing folder, and open up the petz 5 version of the item you based it on in resource hacker.
go to 'action' and 'add from a resource file'. select 'overwrite' for duplicates again, but manually select all resources EXCEPT FLH/FLM as they are not used by p5.
this unfortunately means you'll have to re-do tinker shelf images and such for p5 versions, as far as i know there is no way around that :(
assuming you still want a custom shelf-image filmstrip that'll show up ingame though, we need to change the sprite names. this can be done within resource hacker, simply open up every resource and rename them to something of equal length.
for example: clot_hatbunny -> clot_hatfunny
make absolutely sure to also change it within the 'string table' entry, and click 'compile' when prompted about changes having been made to this script.
after that's done, 'save as' and overwrite your p3/4 version of the file again, keeping the exact same filename.
now open up this newly saved file in your hex editor. do a 'search and replace' for the original sprite name and replace it with your new one. make sure to let it replace all instances.
save, and then open the file up in tinker to reinsert your filmstrips. you have to do this by hand as to my knowledge you cannot convert FLH/FLM to SPR format, but oh well... on the bright side, petz 5 actually doesn't forcefully compress images into the petz palette, so generally SPR resources look more high quality if you still have original png versions!
after all that's done, your item should show up in petz 5 with an unique sprite on the shelf!