There is no built-in command to build this meshL from the 3d mesh. What you can do is copy by hand the 1d data from the file .mesh and put them to another file, so that you can read it with FreeFem to build the meshL, by adapting to x,y,z components the example of Input Border Data from an External File in FreeFEM++ - #3 by fb77
which has only x,y components.
At the end instead of mesh Th=buildmesh(a(nborder));
you can do meshL ThL=buildmeshL(a(nborder));
Anyway I don’t see what you want to do afterwards, since in FreeFem line integrals cannot be used in variational formulations for 3d finite elements.