How to handle video with different frame rates in Lightworks?
Looks like you will need to transcode all clips with frame rates not matching the project's frame rate:


  If the frame rate is of a clip is not compatible with your project, it
  displays in red.  You will be unable to import the clip into your
  project.  Clips with incompatible frame rates may be playable in a new
  project set to the  same frame rate as the clip.


(page 40; User Guide)

You can use ffmpeg, a command line tool, to transcode such clips with commands of the form

ffmpeg -i input.mp4 -c:v libx264 -crf 8 -r 25 -c:a copy -map 0 input25.mov


Edit: should make clear that 25 should be replaced with the framerate of the project. NTSC 30d i.e. 29.97 fps is denoted as 30000/1001 and 23.976 as 24000/1001.