This works without transferring funds or paying a new XRP account reserve. You even keep your r.. address. This has to be done separately for EACH account in Toast.
1. Open the account details screen and select 'Show Secret'.
2. Enter the passphrase and select 'Reveal Secret'.
3. Copy the Account Secret string (starting with 's')
Now you are ready to import the secret into XUMM. Open XUMM, and:
4. Select "Switch account" in the home screen (upper right corner)
If this is the first account then proceed to next step
5. Select "Add Account"
6. Select "Import existing" » "Full access" » "Family Seed"
Select "Import existing account"
Select "Full access" account type
Select a "Family Seed"
6. Enter the s... code copied from Toast
Now type or or paste your "Family Seed" (the code starting with a lower case "s") you got from Toast, in Step 4.
Now PLEASE CLEAR your clipboard
It is possible to use the XUMM camera function by restoring the backup code to a browser version of Toast revealing the secret there and scanning the QR code by selecting the rectangle on the left of the Account Seed input field in XUMM.