QlikView doesn't offer 'write back to source' as a feature. It is an extensible platform however so it is possible to write macros or extension objects that extend the out of box capability to perform the write back . Its also possible to send a list of values via a parameterized URL to an external web service (which handles the write back) . These solutions require a developer skill set to complete .
QlikView offers 'what if' scenarios which will allow users to input values as variables to forecasting. This isn't quite the same as 'write back' as it lasts only as long as the session... temporary write back if you will.
Lastly , there is an option to 'open databases in read/write mode' , but that is just at the connection level.
