We did have to do one at a previous job but it was hard work.
The data had been converted into AutoCad but we had difficulty getting that into SE because of model positioning and format problems. (It was a very large site with a model datum several km from the actual scan data)
The way I eventually did it was to download a trial of SpaceClaim, import the dwg into that, move it and then export it as STEP which went into SE quite easily.
I think there have been several posts on this before and I suggested Sycode Software which has tools for importing and converting various formats. I can't say if or how well they work but they do allow free trials.
bc.
2.4GHz Core2 Quad, 4GB RAM,
Quadro FX4600.
Where would we be without sat-nav?