Currently the datatype mapping is done via an internal Datatype.xml file, containing a list of all supported data types (mappings). It would be nice if somehow (also via designer) you could add your "custom" mappings.

I'll give an example of why this would be handy:
I had to connect to an existing sqlite database via entity framework. This database was created with "custom types" like UINT64, UINT32, UINT16 ... (which seems to be excepted). Currently these can't be mapped and will therefor not be added while importing.