Hi Sebastian, There is indeed no standard Kentico solution for this, so you need to build an custom form control. We build one similar in Kentico v7, to add children (name + date of birth) to your profile. We save the data in one field, like this :
[{"Name":"Child 1","DateOfBirth":"\/Date(1126396800000)\/"},{"Name":"Child 2","DateOfBirth":"\/Date(1172534400000)\/"},{"Name":"Child 3","DateOfBirth":"\/Date(1242086400000)\/"}]
If you want I can send the code to you.