I wanted to add more items but it wouldn't let me. :(

The item 'Magical Keepsake Box' can be bought for 150,000 sP in the Millionaire Center. It gives you at random anywhere from 1 - 3 new treasure chest spaces c:

You can buy them from the Millionaire Center once a day, or in user shops. If you use one on your pet it will give you 1-3 additional spaces for your treasure chest.

, 1-3 treasure chest slots will be awarded when opening a Magical Keepsake Box for a pet. The box can be found in millionaire center (150k).

, you can get at least 10 slots by using a Multi-Compartment Keepsake Box. But it's really expensive,and I've never used it before.

Thank you!

i just get mine from random events, they happen quite often lol

Lucky! :P