In Vim , how can I copy from several lines? (not a range);

You can append to a register instead of erasing it by using the upper-case letter instead of the lower-case one.

For example:

:1y a      # copy line 1 into register a (erases it beforehand)
:3y A      # copy line 3 into register a (after its current content)
8G  # go to line 8
"ap        # print register a