Finally I found the solution.
Simply surrounding the 'cell data' with a double quotation mark is enough and Excel will handle them as one cell.
string clipboardData= "\"Col A\"\t\"Col B\n2cnd\"\t\"Col C\"\r\n";
If you copy that to clipboard and paste it in Excel than you get this:
+-----+-----+-----+
|Col A|Col B|Col C|
| |2cnd | |
+-----+-----+-----+