The best way is to use the Silverlight controls provided by SyncFusion; in case it's not available then it's going to be very tough to port a third party WPF control into SilverLight, it's obvious from the fact that 3'rd party itself is not providing that control.
You will definitely have to look into the source code and port that into SilverLight, but beware of licensing and legal things.
If you can provide details about control you want to port someone might be able to provide you an alternate solution(open source or 3'rd party) for this, that will be the easiest solution for you.
In case you decide to port the control these links will be useful -
Porting from WPF to Silverlight: The Missing Pieces - Part 1,
Part 2
WPF Compatibility
Programmatic DifferencesBetween Silverlight and WPF - A Whitepaper by Wintellect, LLC
XAML Processing Differences Between Silverlight Versions and WPF