You can set the stem direction of any selected notes (provided they are a sort that have stems) using the three Stem Direction options on the Notes menu. The options allow you to either fix the stems up or down, or revert to the stem direction that Rosegarden would otherwise have chosen. See also Beams.
The menu also contains options for adding slashes across note stems, for example to indicate short repeated notes or drum rolls. These are currently purely notational; Rosegarden will not take them into account when playing the staff or generating MIDI.