Is there a way to enter breastfeeding?

If you mean logging your breastfeeding for the purpose of stats, then you could create a custom acitivity, name it Breastfeeding, and start/stop it in the same way as any standard sport activity. You would then see it in the Calendar, as well as in the list of activities.

I was more interested in it adjusting the calorie numbers for it as it should be 500 to 600 calories a day higher. 

Ah, OK, in that case you could do it by editing the Calories manually in the custom activity, or you can do it also through MyFitnessPal:

https://support.myfitnesspal.com/hc/en-us/articles/360032623451-If-I-am-breastfeeding-or-nursing-how-do-I-adjust-my-goals-

Thanks!