in fact, maybe it's easier if you instead just write a batch file to do the job, something like this
copy %1 levelpak.dat
custlemm.exe
If the batch file is called, say, custlemm.bat, you can launch it like this in DOSBox
custlemm.bat mypack.dat
[note: you need the .bat since there's already a custlemm.exe. Or you can choose to rename custlemm.exe to something else]
And it will run the batch file, which copies mypack.dat over to levelpak.dat and then run custlemm.exe
DOSBox should support batch files.