You can use Float's Spreadsheet Data tool to easily copy and paste your forecast from elsewhere into Float. This option can be found via the Blue 'Add' button at the side of the table.
While you can't upload directly from a spreadsheet, you can certainly copy your budgets right in.
Set a budget for any date within a month by using the date selector:
Make sure the numbers are formatted without any symbols or commas, then use ctrl + C and ctrl + V to paste rows from your spreadsheet in and hit save.
This can be repeated for budgets set for a different date.
You can also use Spreadsheet data to move between cells using your keyboard’s arrow keys, making it quicker to enter budgets. Additionally, you can click and drag a cell to the right or downward to copy values across.
Spreadsheet data is currently available in the 'Base' scenario but not on scenario layers