In principle, you could go over USB using a USB to PCI "reverse bridge". BRI is slow, so it should be fine over a USB connection. There are chips that do the USB to PCI function such as the PLX NET 2282 but I don't know if there are boards available that use this or other chips so that you can just plug a couple boards together.
Even if you can find the hardware, getting things to work at the device driver level could be difficult. Good luck!
Maybe with than http://www.xmos.com/startkit they already have lots of pcie for the raspberry pi.
If I understand their documentation correctly, XMOS startKIT has a PCIe connector, but the signals aren't PCIe. They're just using PCIe as a convenient, inexpensive connector.
Instead of going with a ISDN PCI card, why don't you go with a ISDN to SIP gateway like a MediaPack 408, ISDN VoIP gateway. I am not endorsing this device but I have had pretty good luck with a MediaPack MP118, PSTN to SIP gateway. I have also been looking into moving from an old desktop, converted to Asterisk, to a PI. As you can see, I don't have to worry about any PCI or PCIe. :-)
I have a simple question.
I want to use raspberry pi to create a telephone exchange.
I need to use an ISDN BRI but i only can find PCI or PCI-Express boards.
How can I put a PCI/PCI-E on Raspberry Pi Board?
Or how can I fix that situation using Raspberry pi?