You can apply formatting from a regular PPT file to another PPT file -- you
don't actually have to create a template / POT file out of it.

But here are instrux for creating your own template.
http://www.indezine.com/products/powerpoint/ppcreatemp.html and

http://www.soniacoleman.com/Tutorials/tutorials.htm (creating templates)

You might be able to just delete all the slides in the file and File | Save
As, then choose "template *.pot" from the "save as type" dropdown.