Dear Jean-François,
The simplest way of coupling a 1D to a 3D domain is by means of the "Interpolate" function in the general weak form assembly language in GetFEM. You can add an identity interpolation between the 1D mesh, representing your lines, and the 3D mesh representing your volume, with
add_interpolate_transformation_from_expression
("3Dto1D", mesh3D, mesh1D, "X")
then, in order to couple the two domains, you can use the interpolate name "3Dto1D", or whatever you call it, in any assembly _expression_ integrated on your 1D domain, .
Best regards
Kostas