Asking a complete stranger for a subreview for a conference? This question is about Computer Science firstly, I don't know if it applies to other domains

I am a PC member of some international conference in my field. I got a large number of papers to review, so I need to find subreviewers for many of them. Most of the time I would give the paper to a colleague that I know would be interested in reading it ... but I know only so many people interested in so many different topics, and there remains some papers for which I have no idea of whom could review it.

So I'm doing the following: search the web for any researcher in this domain who is active, has publications matching the keywords of the paper, does not seem to have any conflict of interest with the authors, and, preferably, published to this conference before.... and ask this person to act as subreviewer.

Is this common practice? I have received conference papers from complete strangers before, but this was only rare occasions (say a few percent), but I feel I'm doing this for almost 1/4th of my assigned papers. Is there any way to address this person, other than the standard EasyChair message?
I am a CS guy and can answer this question. 

Yes of course. That is what conference chairs do perhaps. As you told the person has already published in past conferences like this. If some researcher is active in the field of his/her research arena and agreeing to review the manuscript. This is what is called peer-review. My recommendation would be to contact the researchers through academic e-mail ids of theirs to make it more transparent.