who ever supplies your C2W scheme, and i think there are a few private ones linked to the Government one, plus the government one and a few specific to large bike shops, so depends on what one your employer has signed up to,
If its not on there list then no you cannot do it. i would be surprised if a german mail order company was on it to be honest.
To be honest the scheme is no longer good, you will often be charged an admin charge if the bike is in a sale, and after one year the price of the bike is fixed by a chart, so example a £1000 after a year will be worth £750, you will be asked to either buy the bike out the scheme, normally £300 plus, or continue the scheme for another 3 years, by then the bike is regarded as very low value.
The old days of getting bikes really cheap on the scheme are gone.