You can do this only if your settings in SU01 are made as
In Defaults Tab - > Decimal Notation should be changed to X/Y depending on your requirement and re-login and then you can find the changes reflected.
You can not forcibly convert the value in deciamls to commas...even if you convert using some replace statements when you are processing that value it results in DUMP.
If you are using it for display purpose it is fine.
Try using Replace statement.
