i am just wondering is it possible to convert a continuous servo to standard servo?
Sure, just add a feedback loop for the position.
Arguably a servo system has this already - but not necessarily to control position.
OP : do you mean a velodyne ? - a servo system which holds a motor at a constant speed irrespective of load?
It sounds like you are asking if a continuous rotation servo can be converted to a position (=/- 90 degrees)
Possibly but it would require coupling the shaft to a pot and using that as the feedback signal for closed loop. If you google the topic there is ample information for the opposite of what you are trying to do.
(converting a position servo to continuous rotation)