We have found from our Web Development Department, that there is not a way in the Numbers program to convert a file to Text Tab Delimited. There is however, a way around it. We found the information below on a 'Mac Help Forum'.
1. Copy the Numbers Table.
2. Paste into Pages
3. In Pages, Format > Table > Convert Table to Text
4. Your new Pages file now can be Exported as Plain Text and will be Tab Delimited.
5. If your text file needs a particular extension, change it after exporting.