It is far easier if you start capturing when you start the command, but see Re: Capturing the text in a Console Window for a simple script that will capture all the text currently in a console buffer.

This will include any that has scolled off the top, (up to the limits of the console buffer size), but note that if more has already scrolled of than could be held, you'll loose data. And any that comes after you capture will not be available to you.

You can try doing multiple captures, but then you'll have to try and stitch them together, discarding any that is duplicated in the second and subsequent captures.

It would be far easier to start the command and redirect the output you want into to a file, perhaps via tee.