Hello Uwe,
After speaking with IBM support I was told there was not a way to extend a trial account.You could create a new account, but would need to use a different email address, if you elect to do that all information from your trial account does not transfer to the new account.
Why not use a Lite, account or you can use a Pay As You Go account. If you decide to go with a Pay As You Go acct, be certain you set your Dollar Thresholds!