Can I install a hot water circulator pump without using the drain valve?
You can use the cold water inlet to connect the recirculator input by a tee (if your boiler don't have a half-height water inlet purposed for recirculation return), just put one-way valves on both recirculation return and cold water feed. But usually, boilers has an inlet at about half-height to connect the recirculation return.

NOTES: 


  black rectangle on the left represents the boiler
  you need both the 1-way valves because you:
  don't want cold water do go 'backward' trough the recirculation loop
  you must avoid backflow to cold water supply
  boiler hot water oulet and cold water inlet are on right side but can be on any side of the boiler.