A report was exported to Excel. When using =sum(start:end)
on a numerical column the result is 0.00. But if use =cell1+cell2+cell3
the result is okay.

It's because the numbers are seen as text due to the
import/export, no formatting will change that.

I'm still too new with Excel to be sure of my answers; but, in my test with
numbers entered into cells as text, the following array formula

=SUM(--TEXT(A4:A8,"#."&REPT("#",15)))

entered using Ctrl+Shift+<Enter seemed to work. Obviously, if you knew a
maximum number of decimal places, you could dump the REPT function call and
simply specify it.

Rick