How can I transfer my fax number to Popfax online fax?
As stated in their FAQ you will have to file a number portability form which will permit them to transfer the existing number from your current provider to their system. If available you will find that option during he subscription process.
If you already signed up with them I suggest to contact their support.
Since phone numbers are usually geographically bound not all numbers can be ported. According to their FAQ it's only available for numbers within USA, France and Spain.