You can add new hoods to both LS and PS as long as there made with just the base game.
LS you can put the 3 Base Games Hoods and play them IE: Pleasantview, Veronaville and Strangetown.
PS does not have Elder Life Stage and I have not tested the 3 Base Game Hoods yet but will do so some time later.

Any hood you put into either LS or PS MUST be # N001, N002 and N003 no other number will work.
Do not just copy and paste them in for you will have problems. You must remove the org. hoods out then place the new hoods in.

ADD after some testing: As long as you add new hoods from N004 and above they will work in both LS and PS games.

ADD again: After more testing you can named your hoods the same way you can in Sims 2 for my "The Begging of Strangetown" hood which is STR1 works great in LS.

I'm guessing this doesn't work with Castaway Stories...? I'm stuck with two islands forever.

Shame that you have to remove the original hoods for it, though. Oh, whyyy didn't the Stories games came with neighborhood/island creators?!

I have PS and I didn't have to delete the original neighborhoods. I downloaded the Monopoly game town (the empty one with only the roads and one lot), renamed the folder to N004 and placed it in the Neighborhoods folder. It worked, and all the original three neighborhoods are still there along with it.

Just WOW it works, I wonder how many it can hold that way. I will now have to rewrite my post on this after I go test it out on how many hoods you can use and see if it will work in LS to.

ADD: Have went a did some testing on this and it works for both LS and PS, as long as start with N004 and above they will work in both.

Have only tested this with base game hoods.

ADD again: After more testing you can named your hoods the same way you can in Sims 2 for my "The Begging of Strangetown" hood which is STR1 works great in LS.

I'm playing Pleasantview in Pet Stories. There's supposed to be no elder stage in PS, so Mortimer Goth became an adult, although he still looks like an elder.

If I want to play Strangetown in PS, do I have to download a clean hood or is it already there and I have to somehow set it up? I would also have to name the folder N004? Oh, does this mean empty hoods can be place? (meaning empty but with roads) Thanks!

Pet Stories doesn't come with Strangetown. You can download a clean hood. I got a clean Pleasantview from here . The other hoods are also there. Yes, you can download empty hoods.

For the neighborhood to work properly, you have to:

-rename the neighborhood folder to N004 or above, depending on how many hoods you have in your folder. They must follow the sequence. N001 must be followed by N002, N002 by N003, N003 by N004 and so on. If not, they won't show up.

-rename the neighborhood's package file and the picture inside the folder to N004_Neighborhood

-rename all the lot files inside the Lots folder. If lots in Strangetown are named N002_11, N002_12, N002_13 and so on, for example, you have to rename it to N004_11, N004_12, N004_14 and so on. It's a bit tedious but if you don't do this the lots won't show up. The neighborhood will show up, but it will be empty and have no lots in it.

This is not right for you can download the 3 hoods in my sig and play them in both PS and LS and the Strangetown one is numbered STR1.

For the hoods here http://meetme2theriver.livejournal.com/63030.html just remove the N001, N002 and N003 for which ever one you want to play if you don't want to renumbered them.
You can use Pleasantview Strangetown and Vernoaville in both LS and PS no other ones will work except maybe Bluewater Village may work in LS. I have not tested this out yet.

ADD: Bluewater Village will not work. So far only base game hoods work in both PS and LS.

Hmm, would LS neighborhoods work with PS? I don't have LS, but this came to my mind - if it does, it could be nice to have more neighborhoods.

Well I will just go test that BRB.

Back from testing this out and no LS hoods will not work in PS game. It has the same problem when you put LS hoods into Sims 2 they just wont work.

I am just too lazy.I bought PS and CA and LS but never got to play it.Well I guess now is the time for me to do so