If you really need to do it this way, you can just copy the files that get installed globally directly into your virtualenv. For example I couldn’t get pycurl working since the required libraries weren’t installing, but apt-get install python-pycurl did. So I did the following:
sudo apt-get install python-pycurl
cp /usr/lib/python2.7/dist-packages/pycurl* ~/.virtualenvs/myenv/lib/python2.7/site-packages/
The install said it was adding it to /usr/lib/python2.7. So I looked in that directory for a site-packages or dist-packages with pycurl, after looking at the files I copied them into my virtualenv. You’d have to also copy any executables from bin into your virtualenv’s bin directory.
Also, running a pip install -r requirements.txt successfully found pycurl in there and just skipped over it as if I had installed it via pip.