It seems that byReadData is an array of UDINT or DINT.
The conversion brsitoa is correct.
If you want to add the newline $n to the generated string, you could use an additional brsstrcat.
Then you have to search for the delimiter ‘;’ in this STRING with for instance the FIND function from the STANDARD library and then use functions like LEFT also from this library.
In the language c or c++ you could also extract these numbers with functions like strstr or strtok.
CSV files are ultimately pure text files. You can simply read the data into strings, but you have to separate the individual fields yourself which might be a lot of work.
You may save yourself a lot of work if you use this library: