Hello Andy,
It might be that there is no conversion possible in S7. But most of the time when it could be presented in S5 as LAD then it must work in S7 the same.
The conversion is from STEP5 to STEP7 in ASCII. After that you have to compile this ASCII file and then the blocks are presented. Because the ASCII-file is a STL file, the blocks will be generated in STL. In the editor how ever you may switch them under "VIEW" back to LAD or FUP.
So you have to switch the view in the right 'language'and then it would presented in LAD, save it and the next time it would directly presented in LAD.