Normally, new files should be added to the branch and not to the trunk (see previous FAQ). This is by design because adding a new trunk file to an existing branch is like "rewriting the history". The whole idea of version control and configuration management is to present the project history as is.
The best method is to delete the file from the trunk (twice) and add it to the branch. The disadvantage of this method is that all the file's history is lost.
If you do not want to lose the file's history, you can duplicate the file in the branch and in the trunk. The disadvantage of this method is that all changes performed to the branch copy must be merged manually to the trunk copy. The rest of this FAQ describes this method.
You may not add files to a branch with names which already exist in the trunk. Therefore, you should rename the file or move it to an alternative folder and then add it to the branch.
For example:
When the file in the trunk is C:\Project1\MyFile.txt, you should add this to the branch as C:\Project1\MyFile1.txt or C:\Project1\Branch\MyFile.txt.
When you perform a branch merge, files added to the branch are added to the trunk. Therefore, you should then select the branch file (e.g. MyFile1.txt) in the trunk and perform Delete.