"Project ID" is just a text field though - so it can be anything (e.g. sourced from your external system perhaps?). ( The internal id (the 5000000 number) doesn't appear in the XOG at all so you don't worry about it )
You might have chosen to auto-number the "Project ID" in your system though, that is your choice. If you wanted to preserver that sequence in your "imported" projects then your problem is really how to build the logic to select a new value from the application's auto-numbering sequence and not really a problem with SOAP/XOG as such.
Actually some XOG allows you to send in an ID of "-1" which tells the system to use the relevant auto-numbering sequence, offhand though I don't know whether the project XML supports this - you could just test it perhaps?
In short - yes everything you want to do is possible,