I made some progress, but it is taking time,
on ISPConfig 2, I managed to export everything in one text file. I could not use SOAP or XML files, because I got errors.
on ISPConfig 3 side. I can remote connect and call functions.
First problem is field names exported from v2 does not match natively to v3, so I have to make a lookup table, currently I am doing this.
there is no function to create resellers in v3. this is not much problem, I can create them by hand.
I could not see functions to query clients, domains with some fields. For example If a client with same name exists, I need to update, if not add. In remote API I could not see a function to check this. Although I can query database.
It seems that, this will take some time but, it will be done