No, it is not possible (at least not at SourceTree 1.4.1.0).
It is possible however to do it manually by editing the file /.git/config in your git repository.
The Git-Flow extension adds it s configuration in the following two sections, which can be freely edited.
[gitflow "branch"]
master = master
develop = development/current
[gitflow "prefix"]
feature = feature/
release = release/
hotfix = hotfix/
versiontag = release/
support = support/
Resource: http://www.crisi.ch/change-git-flow-settings-in-sourcetree/