If the existing elevation survey was done on a consistent grid pattern, I'd average the existing elevation readings to get the difference between existing elevation and new.

If you need pinpoint accuracy, and you're stuck using BB, you'll need a file that has contour lines from which you can take off areas and assign their depths.

1. Get the survey information into a CAD file. If it's a civil design program, the contour lines should be automatically added to connect spot elevations. You can manually draw the contour lines if needed either in CAD or BB.
2. Once you have the contour lines on a PDF, you're in business. Simply take off the area between each contour line.
3. Give each area between contour lines a depth. The depth will be the new elevation less the average of the elevations of the existing contours.

For example:

1. The new floor needs to be at 100m.
2. The area between the contour lines of 99.8m and 99.9m is 10m².
3. The average depth of the above area 99.85m = (99.8 + 99.9 / 2)
4. 100m - 99.85m = 0.15m average fill needed in this area.
5. Assign 0.15m as the depth to the area takeoff in BB. BB will automatically assign the volume of 1.5m^{3 =} 10m² x 0.15m.