You can insert data from a text file or from the clipboard into a worksheet by first selecting the cell where you want the top left item of the inserted data to appear, then choosing From Text File... or From Clipboard... from the Insert → External Data sub menu.

In both cases Calligra Sheets will assume that the data is in CSV form and will open a dialog box allowing you to control how the data is extracted from the file or clipboard and placed into the worksheet cells.

If support for it has been included in your system, Calligra Sheets can also insert data from a SQL database into a worksheet. This is done by using the Insert → External Data → From Database... option.