How do I get logged in users data into a form?

Hi all,

I have created a signup form which is only visible to the logged in users.
What I would like to have is for the form to automatically get that user's username and email address and store that in the form along with other data they need to fill in. Preferably have it as unchangeable or hidden fields.
Is this even possible? Maybe by getting the information from the current session or so?

Currently I have it as open textfields where the users need to fill that in themselves, but I have no way of checking if it really is that user when the form is sent in.

Have searched the forum, did find some reference to this not able to make sense of it all. And then ran into the message that that thread on the fomr module was discontinued.
http://forum.websitebaker.org/index.php/topic,2221.msg21193.html#msg21193

I hope the above makes sence. If not, let me know and I'll try to explain more.

Thanks in advance,

Milagh
Keep up the good work!