I would like to create a web page to let user add disk with a different storage profile to a specific VM.
User could have a dropdown list to select vm then specify storage profile and size of the new disk.
Can someone give me little help or examples on how do to that?
You should use the vCD API. Just follow the documentation VMware vCloud Director 5.6 Documentation Center
In a nutshell you should send a POST API call to VM's "disks" section and provide neccessary section in XML.