You can only do conversion between LAD/STL if you do it on a 300/400 controller in the TIA project, not a S7-1500. At least not in TIA 13/14, haven't bothered unwrapping the TIA 15 disks yet.
If you have STL specific instructions (like memory swapping) or double assignments in the same part, you can't convert it either.
But something like a simple
LD M0.0
A I0.0
= Q0.0
is fine.
I have made two projects now with precise motion control written in ladder. Ended up with multiple function blocks written in SCL and just tossed it in there to overcome the insanity. Never again.
And just you try to convert a function block written in STL or LAD to another platform.