How can I enter an activity (manually) with heart rate transferred?

You cannot. Better told you could do it by hacking a TCX or GPX file manually, but that would not be an easy task for someone not familiar with such work. However, even if you entered the HR data manually, it wouldn't have any impact on any metrics like the Training Effect, VO₂max, etc. so it would be a completely wasted time anyway. Better always record your activities on the device, there is no reason for adding them manually.