PCI-E power and EPS12V connectors appear similar, but there are important differences:
- EPS12V has four ground pins on 1-4, and four +12V pins on 5-8, possibly from two different rails.
- The PCI-E power connector has +12V on 1, 2, and 3, ground on 5, 7, and 8, and sense lines on 4 and 6... almost the opposite.
Take a look at the two connectors here. See how the EPS12V has yellow wires (+12V) coming into the "top" of the connector (the side with the clip), while the PCI-E cable has black (ground) on the top? Right.
Since these are not the only connections to the PSU's +12 and ground in a typical computer, plugging a PCI-E power cable into an EPS12V receptacle, or vice versa, would almost certainly put a dead short across the power supply's output.
To keep you from making this mistake there are subtle keying differences in the connectors (in the shapes of the plastic insulation around each pin).
Therefore, even if you have a spare PCI-E cable for your modular supply, you will not be physically able to plug it into your motherboard's EPS12V connector. Nor would you want to.
An adapter would be possible. I found one: 1st PC Corp CB-6M-44F 12" 6-pin PCI-Express male to 4+4-pin EPS female From the picture you linked of your PSU the PCI-E output connectors are of the 6+2 type. So you could get that adapter and just plug the 6-pin PCI-E connector into it, then plug the EPS12V end of the adapter into your motherboard.