That is fundamental to spreadsheets, cells are separated. It is not copy adding the separation, it already exists.
In writer, use Find and Replace with Regular Expressions to replace tabs with spaces.
In Find enter \t
In Replace enter a normal space with spacebar key
Tick Regular Expressions
Click the Help button on F&R dialogue for more information
In Notepad the tabs are converted to four(?) spaces by Notepad. You could copy that text to Writer and Find and Replace four spaces by one space, or more generically replace two spaces with one space until there are only single spaces left.
[Edit]
The easiest way is enter in cell 4 (assume A4) the formula =A1&" "&A2&" "&A3
and copy just cell A4