How to customize the taskbar?

Hello,

I want my users to be able to send me a feedback (on the webmail)
directly by clicking button in the taskbar. So I added a line into
skins/MY_SKIN/includes/taskbar.html:

<roundcube:button command="compose" label="feedback" class="button- feedback" />

into skins/MY_SKIN/common.css

a.button-feedback { background-image: url(images/buttons/feedback.gif); }

And it is sort of working now, but I would like to prefill an email
adress and a subject on the composition page - is that possible to do
through the file taskbar.html? Thanks for your advices and suggestions.