The 27-year-old played for the Rangers' reserves on Tuesday in their 3-0 defeat against Reading.
Volz will be boss Mick Harford's fifth signing at the club since replacing Paul Hart last month.
Writing on his blog recently, Colz said: 'I'm very grateful they're giving me the opportunity to build up my match fitness after such a long time out. You can train all you like but it's matches you need to really get back to your best.'