What you are requesting is normally a router/gateway function, i.e. "QOS" and/or "Port/IP Prioritisation". However unless you have a high end router (or a hacked one running something like Tomato/DD-WRT) these functions are normally only on expensive routers. I do not know how to do it on the Syno box but would be interested if anyone knows as my WAN syncronisation of syno to syno can eat all my upload bandwidth very quickly resulting in my normal internet use crawling along.
Dont forget the modifications wiki area at http://forum.synology.com/wiki/index.php/Modifications
DS412+ (4xWD30EFRX in 2x RAID1) on LAN, CS407 (4xWD10EADS in 2x RAID1) on LAN, QNAP TS-119 (1x HD103UJ) on LAN, DS212j (2xWD30EFRX) on WAN, DS210j (2xWD10EADS in RAID1) on WAN, DS207+ (2xWD5000AAKS in RAID1) on WAN, ipkg, Y-CAM Black, APC Back-UPS ES 700, Sonos, Media Monkey, Zina, Samsung S3.