If you're just receiving a DXF with "outside lines" that's not going to be very helpful - because NC routs/slots require the center-line and path diameter. Outlines look great for DXF documentation, but are not very useful for re-constructing rout/slots. If you have DXF files with just outlines, I would recommend reconstructing the slot center-lines (in a CAD tool - like AutoCAD) and create polylines (1.6mm wide) that represent the tool path. Import the DXF (setting the DXF layer option "Wide Polyline" to "round" ), and then use the command command Convert to Mill Layer... (see below).
Note: CAD tools are best suited for these type of geometric edits, however FAB 3000 may be a sufficient CAD tool replacement. Here's a quick tutorial (no sound) showing me use Offset to reconstruct the DXF outlines to get the centerline-path width,
Note: Again, once you have the round traces on any layer FAB 3000 can convert to a Rout (see below).
Convert to Rout/Mill Layer: If you have Gerber traces than just use the command Convert to Rout/Mill Layer... (menu: Fabrication | Generate Drill/Rout Layers). This will convert any Gerber traces or flashes found on the layer to Routs and Drill hits respectively.
Here's a few references from the help center,