BAPI_ACC_ACTIVITY_ALLOC_POST vs. ACC_ACT_ALLOC01

Hello,

I would like to use

ACC_ACT_ALLOC01

IDOC for posting activities like in transaction KB21N. Problem is I can't find any Processing code for that in WE20?

Is there any or should I made new processing code in WE42, which uses

BAPI_ACC_ACTIVITY_ALLOC_POST

FM?

If there is ready made easy solution to post activities from other system using Idoc and some processing code I really like to know.