The Basics:
How to I convince colleagues that we should send time filtering/interviewing people when we aren't currently hiring? I'd like to ask the community for advise on how to build a bench. I can certainly go to networking events and meet A-players through friends and social life that I consider to be on my bench, but I'm wondering as to how I can go about filtering/vetting individuals that apply to my company but which the company doesn't want to hire.

All the background details:
I am the default HR person as a 20 person company in China. We are a culturally mixed team, and we hire both Chinese citizens and foreign citizens (mostly in their 20s) as developers, operations staff, and marketing & sales staff. My inbox often receives either a) unsolicited "general" applications, or b) applicants that are unqualified for one position but who might be a good fit for another position.

For candidates that apply for a position, we interview them, they meet all the criteria, and we decide not to hire them because someone else was a better choice, I'd like to let these candidates know something along the lines of "although my company isn't going to be moving forward with your application, I think that you are a great candidate and that you'd contribute a lot to this company. I'd like to reach out to you when we are next hiring for this position (be that six, twelve, or twenty-four months) to see if you are interested/available at that time. Is this something that you would be open to?

For candidates that are applying generally and whom we haven't interviewed, I'd like to tell them something like "The position that you’ve applied for has recently been filled, and consequently Vericant isn’t currently hiring for this position. However, I reviewed your application with a colleague and we are impressed by your background and skills. If you are open to the idea, I’d like to schedule time to speak with you as something of a less formal interview. Rather than interviewing for a current position, this would be more like interviewing as a sort of 'pre-approval' so that when a similar position with Vericant opens up in the future (whether than means in three months or in three years) we can reach out to see if you are interested/available at that time. Is this something that you would be open to?"