Generally speaking brass extension nipples may be used to install/lower/position fire sprinkler heads, however, in some cases, their friction loss must be included in the design hydraulic calculations, check with your edition of NFPA 13 and your AHJ.
Below is photographed a fractured brass nipple, leak occurred after sprinkler head was installed. CPVC assembly is also shown:
Close up photo of the fractured brass extension nipple follows. Photograph taken through a hand held magnifying glass. Failure likely occurred as a result of excessive torque loads applied to fire sprinkler head during installation: