how to set StartPos (like in dataport) in CodeUnit?

Dataport does not allow you to read records accross more than one line unless there is some other record seperator you can set up.
However, reading from fixed positions is easy from C/AL using file.SEEK or counting bytes read and reading "filling" bytes to a temporary buffer.
You cannot reuse the code for reading though, you need to program writing again from scratch.

Uhm filling with spaces... How about the function PADSTR???