Hi Sachin,
As I see you have already looked into this FM below.
BAPISPP0002_SDL_MODIFY_MULT
You should be able to use this and create schedule lines..Infact we have used this to create lines with an input of product, location, means of transport, Schedule agreeemnt, availability date...
So I suggest you please check with your ABAPer to look into this again.
Best regards,
Binod Sahu