You can't combine the two bookings into one. The only way around it would be whether you can re-route your domestic ticket and pay change fees etc. In the end, this would not be an economically prudent measure.

You can link your PNR's, although it's not really necessary if the same passenger(s) are on both bookings. If you make check-in aware you have an onward connection, they'll check you in for that flight without a problem so long as you don't undercut any connection times.