Converting TIFF file to LAS file in QGIS?

You cannot convert geotiff to las, however, LASTools las2las allows converting ASCII (.asc) format files to las format (source). For example:

las2las -i in_data.asc -o out_data.las

PDAL can convert any GDAL-readable raster to LAS or any other point cloud format it can write using the readers.gdal stage. PDAL is available on Windows via OSGeo4W in addition to Linux, OSX, and Docker.

pdal translate in_data.asc output.las --reader gdal

Heikki's point about whether or not it makes much sense to do this still stands. With PDAL's reader, you're only going to get an X, Y, Z -attributed point cloud, it is going to be discritized to a regular grid, and it will not have any multi-return information in it.