The only way to import data (without adding an Applescript) is to open the source data file in a new, temporary, document copy the data, paste into your template and close the temporary document.
The purposte of the template is to allow you a predfined set of table and calculations that are formatted the way you want.
If you have Excel, what usually works well here is this: select the cells with the data in Excel, command-c to copy, click once on the upper left cell of the range where you want your data to appear in Numbers, then command-v or option-shift-command-v to paste. Numbers will expand the table as needed.
If you are not bringing the data directly from Excel on your machine, but you are getting the data from elsewhere, then if you have a choice, get the data in tab-separated format. That pastes readily into Numbers, after opening it in a temporary document and copying as Wayne describes.
Comma-separated data or Excel format can be opened in a new Numbers document, as you not, but data needs to be converted to tab-separated data before it can be pasted into an existing Numbers document.
If you can only get the data in comma-separated format, then there is an AppleScript that will do the conversion for you. Post if you need that.