First supposing what you mean by pre-populate fields is let user to insert data into your database tables using some form then show this data in a report.
Reporting Services is not designed to insert or alter data anyway. Instead it is used commonly for reporting purposes, to show data using charts and controls allowing limited user interaction.
If supposition is wrong and you want to add a drop-down list to filter your data or specify presentation conditions you can use a parameter
. Go through this technet tutorial where adding and setting for a report parameter is explained.

It is possible to add custom report items to your report, go to this documentation and check this sample in codeplex.