Sure, they will add a small amount of value - think of any additions you make to your method as "more hooks in the water" (to use a fishing analogy).
But to truly use a private blog network for ranking, those blogs will need to already have some value. In other words, a history of content having been posted on them, traffic, backlinks, etc. Also, I would suggest that if you add more blogs, put them on different IPs - either by buying multiple shared hosting accounts (at different hosts) or you could even get a VPS/dedi and a block or two of IP addresses.
I *have* seen site networks that were able to prop themselves up quite well and exist only on one IP - but these are sites that have been around forever and have a truly valuable network on interlinking sites.
There is no single solution for ranking (due to differences in niches, competition, etc) but three factors that never fail (in any SEO strategy) are: value, consistency, and variety.
