How do I make an object placeable in the Sim's personal Inventory? Nothing in the OBJD references it that I can see.

It's a scripting thing. The object needs the ItemComponent called SimInventoryItem.

Usually it's added to objects with the OnStartup method, but I think you can also
add it to existing items with a pure scripting mod. What did you have in mind?

Thank you for your help Arsil, you're very kind. Unfortunately, I'm a newb at modding and I have no idea what you just said. What does "OnStartup method" mean?

Update: When I googled, I found this information regarding inventory items ---

"In s3oc, open the package, click on "clone or fix" and click the "Details" table at the top. There should be a bunch of different boxes that say something like "0x00000000". Under the one that mays "Moodlet Score", there are five different boxes. In the one that says "Topic/Rating" or box number 2, in the left-hand box type the code "0x00000018" and in the right-hand box type "0x0000000B". Click start. Your object will now be portable."

I never used them, but I think those fields are used to specify the features of an object
(for example how much energy a bed recovers) displayed when you select that object
in the catalog (the real values for those behaviours are specified elsewhere though).

EDIT: If you use S3PE's Grid on a OBJD you can see exactly what values those fields can get.
Oh yes, 0x18 stands for "portable", but it doesn't make the object portable, it's only to display that info ^^
The small stereo has that, but what makes it really portable is having that specific ItemComponent.

OnStartup is a method called to initialize (for lack of a better expression) the object.
It's most commonly used to add interactions to an object.

Well, in this case (making the object portable) the method I posted actually worked. The object can now be dragged to the Sim's inventory. Maybe it works for some of the behaviors but not others then?
Now I have to find out how to change the object's thumbnail when it's in the inventory (it displays the thumbnail of the object it was originally cloned from). I appreciate your help, Arsil.

It worked? Oh, well, good for you.

Is there one or more THUM resources in the package?
If yes, extract them, edit them with an image editor and re-import the modified version.

If not you can create the thumbnail yourself: just find and extract a THUM from somewhere else,
modify it, import it and then change it's Instance Id to match the one of the OBJD resource
(make sure the Group IDs also match).

I figured out what was causing my object to show the thumbnail from the original (cloned) object -- I still had the original object in the Mods folder and its instance number was being used instead. -_- oops
The correct thumb now shows up in the inventory after the other object was removed from the game. Thank you Arsil; I appreciate all your help and advice. Cheers, ~pj