I can think of a half dozen methods of convertng the sizes but before doing so you better check your local codes very carefully.

Just as materials must be type accepted under the UL in the electrical trade in the plumbing trade we have a number of independant testing agencies and in the genreral rules of nearly all codes it states that you may not use any pipe, fitting, fixture or appurtenance that does not bear a type accepted approval rating from one or more of the following agencies:

ANSI....American National Standards Institute

ASTM..American Society for Testing & Material

ASSE...American Society of Sanitary Engineers

I'd contact these folks:
http://www.houseneeds.com/shop/atop/heatersbaseboardradiantindex.asp
Casey

Here is a link that might be useful: Euro hydronic heat

Where are you purchasing the towel warmer from?
The internet has made the global market a lot more local, many of the companies will have adapters.

I recently bought Villeroy & Boch Faucets for a customer, which shipped with US adapters - the difference was in the part number.

I would reach out to whoever you are purchasing the towel warmer from. I am sure you are not their first US customer.

If they can't help you ... find out the size of the tappings on their unit.

Euro pipe threads are not tapered, so special pipe dope is a crucial ingredient.
Casey

Did you ever actually get an answer to the question you posted. Lots of helpful info. but it did not appear that your question ever received a direct answer.
I wish to install an German home urinal in our California home to be constructed. The manufacturer does not sell to the States and does not offer adapters, so i am interested in what you learned about adapters.

http://en.wikipedia.org/wiki/Nominal_Pipe_Size

here is an example of the kind of information available to one and all.

Email the German manufacturer to give you the numbers and letters you will need to be able to specify what thread sizes their product's pipes have. Then it's a mere matter of sending that information to a plumbing supply house.