Gee,
It sounds like you've got a lot going on.
I'm guessing that, since you have two Airport Basestations, it would probably be best to just disconnect the internal, omindirectional antenna in one of those and connect a better, directional antenna via a pigtail and short cable, while pointing it toward the hotspot source, via an unobstructed outdoor mounting, if possible.
Then you could connect your Mini or PB via an Ethernet cable. Or, you could then plug the Basestation into those Ethernet-Over-Powerline units. Or, then you might possibly piggyback the omnidirectional Base station, via Ethernet cable, to farm out the signal wirelessly. Or, maybe a USB Wifi card/antenna could spare you some hardware hacking.
I'm just guessing about that stuff, because I've never had any use for a Basestation.
But, I did see an article about hacking an external antenna into a Basestation. You'd at least need a pigtail with an RP-SMA connector (or other antenna connectors) at one end, but the other end would probably be an MC-Card connector (not the IPEX, like on the 2009 Mini), depending on the vintage of your unit.
This eBay seller has lots of this stuff priced well:
http://shop.ebay.com:80/merchant/data-allianceW0QQ_ipgZ50QQ_sopZ12?rdc=1
Be careful about those antenna fittings, they're often counter-intuitive in their gender labeling. Look carefully at the photo close-ups.
Here's an article hacking a Basestation;
http://www.vonwentzel.net/ABS/ExtendedExtreme/index.html
Apparently, some of them have an external antenna jack, already.
Or, maybe that mentioned YDI Max-Ranger antenna from Fab-Corp would do after a minor hack.
The article mentions Dr. Bott and MacWireless products, too.
Do some more research and due diligence, then tell us what you think will accomplish what you want.
But, yes, if you're not grabbing your signal well, a better, external, directional antenna seems to be a primary step for you.
Peace to you,
C. Livingstone