Yes it is possible. Basically, you need to run svn merge
from a clean working copy of your branch (one with no local modifications):
$ pwd
/home/user/mybranch
$ svn status # Does not display anything
$ svn update # Make sure your local copy is up to date.
Updating '.':
At revision X.
$ svn merge url/to/repository/trunk
Updates, additions, deletions and conflicts.
$ #handle conflicts.
$ svn commit -m "Merging changes from the trunk".
See Keeping a Branch in Sync from the SVN book.
The first merge is likely to introduce many conflicts, especially if the branch forked a long time ago, but latter merges will go smoothly, especially if you merge often.