I know that CUTedrale is doing extensions. They are located in frankfurt
bockenheim, große seesstraße 51. near leipzigerstraße.
they work with the company hairdreams - its european hair only and you can use it up to 2 or 3 times.
well even if you are not looking for for extension, its a great hairdresser anyways. very friendly people, great service ( free coffee and cookies), massagechairs, head massage etc. well its not the cheapest hairdresser in ffm- but i just love that place -its like beeing in a spa...
and its very close to public transport.
http://www.cutedrale.de/