Please don’t do this. If you’re using a lime mortar this will compromise the integrity of the mortar. Adding cement to make it set faster will undermine the benefits of a lime mortar and may negate the very purpose it was specified.
Lime mortars set more slowly than a cement mortar by nature but a suitably designed lime mortar mix will set of it’s own accord. Free of cement the mortar will be flexible, breathable and more sympathetic to surrounding masonry.
It’s understandable that this happens in the midst of modern building and tough schedules. When selecting your lime mortar, consider the practical considerations. If you do need a lime mortar and time is an issue you might consider the faster setting hydraulic limes.