You can do this from the Web Vault. Use the checkboxes to select the relevant items (noting that clicking the checkbox labeled “All”, in the column header row, selects all displayed items). Then, click the kebab icon (three vertical dots) at the top (in the column header row, to the right of the “Owner” header), and select “Assign to Collections” from the dropdown menu. This is shown in the screenshot below (including checking “All” to select all items, which is optional):
The best way to do this is to go to the Web Vault, access the Admin Console (using the link towards the bottom of the left-hand navigation menu), and then going to Settings > Export Vault, as shown in the screenshot below:
Unfortunately, if you are using a free organization, then the items stored in the organization vault will not have any most* Premium benefits (i.e., the shared items will not be able to have attached files, use the integrated TOTP authenticator function, or be included in the Premium Vault Health Reports), except that integrated TOTP generation for shared items is now supported for Premium users*.
On the other hand, Premium features such as file Send, Emergency Access, advanced 2FA methods for login (DUO or Yubico OTP) will still work, as long as you maintain your individual Premium subscriptions.
I should also clarify (in case it wasn’t clear), that if you do switch to a paid Family plan (so that you can have file attachments and other Premium benefits for your shared items), then you will no longer have to pay for your individual Premium subscriptions, because Premium status is included for all members of a Family plan. So your annual subscription costs would “only” increase from $20 to $40 (not $60!).
*Edit: As pointed out by @kpiris below, the integrated TOTP authenticator now works for login items stored in a free organization, provided that you have an individual Premium subscription.