You can use Float's Spreadsheet Input tool to easily copy and paste your forecast from elsewhere into Float.
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
Simply select a date. Then enter, or copy and paste in your budgets and hit save.
This can be repeated for budgets set for a different date.
You can also use Spreadsheet Input to move easily between cells and enter your budgets more quickly. And you can click and drag a cell to the right to copy it forward.
Spreadsheet Input is currently available in the 'Base' scenario but not on scenario layers