You cannot put newlines, or tabs, in a cell, in tab delimited files. It comes with the territory.

You could upgrade to a CSV file (possibly even using a tab as the column delimiter), which allows you to quote cell contents and thereby including newlines.

You could also try to just use a standalone return ("\r") in the tab delimited file. No garantees, it could work, but it's an extremely dangerous hack — you never know if somebody editing the file in a text editor could ruin it for you.