@ageekhere: If your intention is to use a cache drive with a proxy server like squid (for example with pfSense), then I'd suggest using a larger mSATA SSD in the designated mSATA slot. Regarding the mPCIe slots: I'm almost sure that these slots can't be used with additional mSATA SSDs, but maybe PC Engines support can clarify this.
No, mSATA drives will not work in the mini-PCIe slots. Although the physical connectors are the same, they are two entirely different ports.
The mini-PCIe ports are connected to the PCI-express lanes of the AMD G T40 APU controller (and USB) and the mSATA port is wired to the SATA port of the AMD A50M FCH southbridge.
You could for instance use a passive adapter in the mSATA port and transform it to a regular SATA connector for a 2.5" or 3.5" drive, such as: http://www.dx.com/p/355291
But if you want to connect SATA drives to the PCIe ports, you'll also need a PCIe SATA controller chip, such as: http://www.dx.com/p/337644
There are PCI express SATA controller cards that have mSATA connectors on board, but these are full size PCI-express (for PC's), such as: http://www.dx.com/p/350661
For embedded systems and laptops M2 SSD's are becoming more common, which can either be SATA drives connecting to the SATA bus or have their own controller connecting directly to the PCIe bus and not use SATA at all (much faster).
The non-mSATA slots should also support USB (like for 3G wireless cards). You could use an adapter like this and load a short/thin usb stick to add storage. Bus speed would be USB 2.0 and capped by whatever USB flash device is used.