Hi Mihalis,
The fundametal basis of your difficulty is that the values of all channels loaded into DIAdem are automatically turned in DBLs, which has only about 52 or 53 bits of resolution outside the exponand. You will have to send your 19 digits to at least two DIAdem channels during loading in order to see in DIAdem all the resolution of the data on disk. This is why Andreas recommended a change in the DataPlugin loading the data file. With a TDMS file, your best bet is to store this information to two separate channels in the TDMS file.
Neither DIAdem nor VBScript have a way to represent U64 data. We're stuck with workarounds.
Brad Turpin
DIAdem Product Support Engineer
National Instruments