Generally speaking, you can use spun articles for fillers for your private blog network.
With that said, if you want to build a high-quality private blog network, you should split up your network in terms of tiers.
For lower quality tiers, filler text, as long as it's completely original, will work.
For inner tiers, meaning these have a better quality backlink footprints and better overall link equity, you might want to consider a higher quality content.
At the very least, look for content that is engaging or aimed at flesh and blood human traffic.